How Fast Can a Honda 450 Go? Unpacking the Top Speed and Performance Factors

Understanding the Velocity of a Honda 450

The question of "how fast can a Honda 450 go" is a common one, sparking curiosity among motorcycle enthusiasts, off-road adventurers, and even those simply wondering about the capabilities of these powerful machines. It's a question that doesn't have a single, simple answer, as the top speed of a Honda 450 is influenced by a multitude of factors. However, to give you a quick and clear understanding, a stock Honda 450cc motorcycle, depending on its specific model and intended use (like motocross, enduro, or supermoto), can typically reach top speeds in the range of 60 to 80 miles per hour (mph) in its standard configuration. This figure, however, only scratches the surface of what these versatile bikes are capable of.

As someone who's spent countless hours in the saddle of various dirt bikes, including Honda's renowned 450cc offerings, I can tell you that the raw numbers on paper rarely tell the whole story. The feeling of acceleration, the way a bike handles at speed, and the specific terrain it's designed for all contribute to the overall perception of its performance. When you're carving through a tight trail or launching off a jump, a bike that hits 70 mph feels incredibly fast. Conversely, on an open stretch of pavement, that same speed might feel like a mere warm-up.

My own experiences have shown me that the journey from zero to top speed is as crucial as the maximum velocity itself. The responsiveness of the throttle, the smoothness of the gear changes, and the bike's stability are all integral parts of the riding experience. Honda's 450cc line has always been celebrated for its robust engines and agile handling, making them a popular choice for a wide range of riders, from seasoned professionals to enthusiastic amateurs. Let's dive deeper into what truly defines the speed and performance of a Honda 450.

The Engine: The Heartbeat of Speed

At the core of any Honda 450's performance lies its 450cc engine. This displacement is a sweet spot for many performance-oriented motorcycles, offering a potent blend of power and manageable weight. Honda has consistently engineered these engines to deliver a strong torque curve and impressive horsepower, vital for achieving and maintaining high speeds. The type of engine – single-cylinder, four-stroke – is key here. These engines are known for their high revving capabilities and the immediate punch they deliver, which is precisely what you want when you're looking for acceleration and top-end speed.

Honda's commitment to technological innovation is evident in their 450cc engines. Features like Unicam valve train technology (found in many of their motocross models) contribute to a more compact cylinder head and a lower center of gravity, which indirectly aids in stability at higher speeds. Fuel injection systems have become standard, providing precise fuel delivery for optimal power output and throttle response across different altitudes and temperatures. This precision is invaluable; it means that when you twist the throttle, you get a predictable and powerful surge, not a hesitant cough.

The power output of a Honda 450 engine can vary slightly between models. For instance, a motocross bike like the CRF450R is tuned for maximum outright power and explosive acceleration, essential for winning races on a closed circuit. An enduro or trail-oriented model, like the CRF450X or CRF450L, might have its power delivery smoothed out for better control in varied terrain and longer riding sessions, which could subtly affect its absolute top speed but enhance its usability. Regardless of the specific tuning, these engines are designed to be potent workhorses.

Key Engine Characteristics Contributing to Speed:

Displacement: The 450cc volume provides ample air-fuel mixture for combustion, generating significant power.
Four-Stroke Design: Offers a broad powerband and efficient fuel combustion compared to two-stroke engines of similar displacement.
Fuel Injection: Ensures precise fuel delivery for optimal performance and throttle response, crucial for reaching and sustaining high speeds.
Camshaft Technology: Innovations like Honda's Unicam system contribute to improved valve operation and engine efficiency.
Exhaust System: The design of the exhaust plays a role in scavenging exhaust gases and can be tuned for power gains, affecting top speed.

Model Specifics: Not All Honda 450s Are Created Equal

When discussing how fast a Honda 450 can go, it's imperative to differentiate between the various models Honda produces. The "450" designation typically refers to the engine's approximate displacement, but the intended application of the motorcycle dramatically influences its gearing, suspension, and overall design, all of which impact top speed.

Motocross Variants (e.g., CRF450R):

The CRF450R is a purebred motocross machine. Its primary purpose is to achieve blistering acceleration and handle the demands of competitive racing on dirt tracks. These bikes are geared for rapid acceleration out of corners and over jumps. While their top speed might be slightly lower in a straight line compared to a street-legal bike with taller gearing, their ability to get there quickly and their prowess in maneuvering at speed are unparalleled on a track. On a closed motocross track, where short bursts of speed and aggressive acceleration are paramount, a skilled rider can certainly push a CRF450R to its limits. In a straight line, it's conceivable that a well-ridden CRF450R could touch speeds in the upper 70s to low 80s mph, but its design prioritizes agility and grunt over sustained highway cruising.

Enduro/Off-Road Variants (e.g., CRF450X, CRF450RX):

Enduro bikes are built for longer, more challenging off-road journeys, often through varied terrain like forests, mountains, and deserts. They typically feature wider gear ratios, a larger fuel tank, lighting, and a more compliant suspension compared to motocross bikes. The gearing on an enduro bike is often optimized for a balance between low-end torque for climbing obstacles and sufficient top-end speed for covering ground efficiently. A CRF450X, for instance, might be geared slightly taller than a motocross counterpart, allowing it to reach slightly higher speeds on open stretches while still maintaining the necessary control for technical sections. You might see speeds in the low to mid-80s mph on these models in ideal conditions.

Dual-Sport/Supermoto Variants (e.g., CRF450L, CRF450RL, or past models like the XR400/XR650 variants that paved the way for broader displacement options):

Dual-sport motorcycles, like the CRF450L (now the CRF450RL), are designed to be street-legal but retain significant off-road capability. This means they are equipped with features like headlights, taillights, mirrors, and emissions controls required for road use. To comply with road regulations and provide a more comfortable riding experience on pavement, these models often have taller gearing than their pure off-road brethren. This translates to a higher potential top speed. A CRF450RL, with its street-legal setup, could realistically achieve speeds of 85 mph or even slightly more on a clear highway, though it's important to remember that sustained high-speed operation might not be its primary design purpose, and rider comfort and wind protection become limiting factors at those speeds.

Supermoto conversions, often built from motocross or dual-sport bikes, typically involve fitting street tires and sometimes taller gearing. These bikes are designed for gripping pavement and lean angles, and their top speeds can also be quite respectable, often exceeding 80 mph due to the enhanced traction and stability offered by street tires.

Estimated Top Speeds for Different Honda 450cc Models (Stock Configuration)
Model Type	Typical Top Speed (mph)	Primary Use	Key Factors Influencing Speed
Motocross (CRF450R)	65 - 80	Closed Circuit Racing	Aggressive Gearing for Acceleration, Power Tuned for Bursts
Enduro (CRF450X, CRF450RX)	70 - 85	Off-Road Trails, Timed Events	Balanced Gearing, Torque for Varied Terrain
Dual-Sport (CRF450L, CRF450RL)	75 - 90+	On/Off-Road Versatility	Taller Street-Oriented Gearing, Emissions Controls
Supermoto (Modified)	80 - 95+	Paved Track/Street Performance	Street Tires, Potential Gearing Changes

Note: These are approximate figures for stock or lightly modified versions. Actual speeds can vary significantly based on rider skill, external conditions, and specific modifications.

The Role of Gearing

Gearing is arguably one of the most impactful factors influencing a motorcycle's top speed. The gear ratio determines how many times the engine crankshaft rotates for each rotation of the rear wheel. A lower gear ratio (more teeth on the rear sprocket, fewer on the front) provides more torque and acceleration, ideal for off-road riding and climbing. Conversely, a higher gear ratio (fewer teeth on the rear sprocket, more on the front) allows the rear wheel to spin faster relative to the engine's speed, leading to higher top speeds on flat, open surfaces. Honda's engineers carefully select the stock gearing for each 450cc model to suit its intended purpose.

For a motocross bike like the CRF450R, the gearing is designed to get the rider out of corners and up to speed quickly on a relatively short track. This means lower gears are emphasized. On a dual-sport bike like the CRF450RL, the gearing is often taller to allow for more comfortable and efficient highway cruising at legal speeds, meaning it can reach a higher maximum velocity before the engine hits its rev limiter or the rider feels uncomfortable.

Changing sprockets (front and rear) is a common and relatively inexpensive modification that riders make to alter their bike's performance characteristics. If a rider wants more top-end speed on their CRF450R for some reason, they might install a smaller rear sprocket or a larger front sprocket. Conversely, if they want more grunt for technical off-roading on a CRF450RL, they might opt for a larger rear sprocket or a smaller front sprocket. This ability to fine-tune the gearing is a significant reason why the question of "how fast can a Honda 450 go" has such a wide range of answers.

Aerodynamics and Rider Position

While the engine provides the power, aerodynamics and rider position play a crucial role in how effectively that power translates into speed, especially at higher velocities. On a dirt bike, which typically lacks fairings or significant wind protection, the rider's body becomes the primary source of aerodynamic drag. As speed increases, air resistance grows exponentially. This means that tucking in, minimizing frontal area, and adopting an aerodynamic riding posture can significantly increase the achievable top speed.

Consider the difference between an upright rider on a CRF450R blasting down a straight and a rider who is tucked low behind the handlebars. The tucked rider will experience less air resistance, allowing the engine to push the bike to a higher velocity. This is why professional motocross and supercross riders are incredibly adept at using their bodies to manipulate airflow and maintain stability at speed. On a dual-sport bike like the CRF450RL, the ergonomics are often more relaxed, but at speeds approaching 80 mph and above, the rider will still be facing considerable wind resistance. The lack of a substantial windscreen means that the rider's physical effort to push through the air becomes a limiting factor.

Furthermore, the stability of the motorcycle at speed is also tied to aerodynamics. Crosswinds or turbulent air from passing vehicles can buffet the bike. A well-designed chassis and suspension, combined with a rider's ability to remain centered and stable, are essential for maintaining control at higher speeds. Honda's 450cc platforms are generally known for their planted feel, which contributes positively to rider confidence when pushing the limits.

Tires and Traction

The tires on a Honda 450 are another critical component that dictates its speed capabilities. The type of tire, its condition, and the surface it's running on all have a profound impact.

Motocross Tires: These knobby tires are designed for optimal grip on loose dirt, mud, and sand. While they provide excellent traction for acceleration and braking in off-road conditions, their aggressive tread pattern can create significant rolling resistance on pavement, limiting top speed and causing excessive wear.
Enduro/Off-Road Tires: Similar to motocross tires but often with slightly less aggressive patterns and sometimes more focus on durability and wider terrain compatibility. They strike a balance between off-road grip and a slight improvement in road manners compared to pure MX tires.
Dual-Sport Tires: These tires have a more aggressive tread than street tires but are designed to be durable and offer reasonable grip on both pavement and dirt. They are a compromise, providing versatility but not excelling in either extreme.
Street/Supermoto Tires: These are slick or semi-slick tires designed for maximum grip on dry pavement. When fitted to a 450, they allow the bike to lean deeply into corners and achieve very high speeds on asphalt due to their large contact patch and softer compounds.

The surface itself is also a major consideration. A Honda 450 will achieve its highest speeds on smooth, hard-packed surfaces like asphalt. On loose gravel, sand, or mud, traction is severely compromised, and the bike will struggle to put its power down effectively, significantly reducing its achievable speed. This is why you'll often see higher top-end speed figures quoted for supermoto-modified bikes or dual-sport bikes on the road, compared to their motocross counterparts on a track.

The condition of the tires matters too. Worn tires have less tread depth and can lose their grip characteristics, making them less safe and potentially slower. Tire pressure also plays a role; too low, and you risk pinch flats and sluggish handling; too high, and you lose traction and ride comfort.

Rider Skill and Experience

It's impossible to discuss the top speed of any motorcycle without acknowledging the rider. A highly skilled and experienced rider can extract significantly more performance from a Honda 450 than a novice. This includes:

Throttle Control: Smooth and precise throttle application is key to maintaining traction and building speed efficiently.
Body Positioning: As mentioned, an aerodynamic tuck can increase top speed. Also, shifting weight correctly for cornering or acceleration is vital.
Gear Selection: Knowing the optimal gear for a given speed and situation helps the engine stay in its powerband.
Cornering Technique: Efficient cornering means carrying more speed through turns, which contributes to overall faster lap times or transit times.
Awareness of Terrain: Understanding the surface and anticipating changes allows the rider to adjust their speed and technique accordingly.

I've seen riders transform the capability of a Honda 450 simply by improving their technique. A rider who is hesitant or makes jerky movements will not only be slower but also less safe. Conversely, a rider who is smooth, confident, and understands the bike's dynamics can push it to its absolute limits and often achieve speeds that might seem surprising given the bike's off-road roots.

Environmental Factors

Beyond the bike and the rider, external environmental factors play a significant role in determining how fast a Honda 450 can go:

Altitude: Higher altitudes mean thinner air, which contains less oxygen. This reduces engine power output, as there's less fuel that can be effectively combusted. A bike will typically have a lower top speed at higher altitudes.
Temperature: While not as dramatic as altitude, extreme temperatures can affect engine performance and tire grip. Very hot conditions might lead to engine heat buildup, potentially causing a slight reduction in power. Very cold conditions can make tires less pliable and reduce grip.
Wind: A strong headwind will act as a significant brake, reducing the bike's top speed. A tailwind, conversely, can provide a slight boost. Crosswinds can affect stability.
Surface Condition: As discussed with tires, the quality of the riding surface is paramount. A smooth, flat, grippy surface allows for the highest speeds. Rough terrain, loose surfaces, or inclines will drastically reduce achievable speeds.

Modifications and Their Impact

The stock configuration is just the starting point for many Honda 450 owners. Modifications are a huge part of the motorcycle culture, and they can significantly alter a bike's top speed and overall performance.

Engine Performance Modifications:

Exhaust Systems: Aftermarket exhaust systems are popular. They are often designed to reduce backpressure, allowing the engine to breathe more freely and produce more horsepower, which can translate to a higher top speed.
Airbox Modifications: Improving airflow to the engine, often in conjunction with an aftermarket exhaust and fuel mapping, can boost power.
Camshaft Upgrades: More aggressive camshafts can alter the valve timing, allowing the engine to rev higher and produce more peak power.
Engine Tuning/ECU Flashing: Reprogramming the engine control unit (ECU) can optimize fuel and ignition maps for aftermarket parts or specific performance goals, unlocking more power.
Big Bore Kits: Increasing the engine's displacement (e.g., to 470cc or 500cc) is a more involved modification that can substantially increase power and torque.

Drivetrain Modifications:

Gearing Changes: As detailed earlier, changing sprockets is a common and effective way to prioritize top speed or acceleration.
Clutch Upgrades: While not directly impacting top speed, a stronger clutch can handle the increased power from engine modifications and ensure efficient power transfer.

Chassis and Handling Modifications for Speed:

Suspension Tuning: Properly set up suspension is crucial for stability at speed. Upgraded or re-valved suspension can absorb bumps better and keep the wheels planted, allowing the rider to maintain higher speeds with confidence.
Wheels and Tires: As mentioned, switching to street-focused tires or lighter wheels can improve aerodynamic efficiency and rolling resistance, contributing to higher speeds on pavement.
Aerodynamic Aids: While rare on purpose-built dirt bikes, adding small spoilers or undertaking bodywork changes could theoretically influence aerodynamics, though this is less common for Honda 450s.

It's important to note that many modifications, especially those that significantly increase horsepower, can also impact the reliability and lifespan of the engine if not done correctly or if the bike is consistently ridden at its absolute limit. The pursuit of maximum top speed often involves trade-offs.

Honda 450 vs. Other Motorcycles

To put the speed of a Honda 450 into perspective, it's helpful to compare it with other types of motorcycles. A 450cc dirt bike is in a performance class of its own, designed for specific environments.

Compared to Smaller Displacement Bikes (e.g., 250cc): A 450cc engine offers substantially more power and torque than a 250cc engine. While a 250cc motocross bike might reach similar top speeds on paper due to gearing, the 450 will accelerate much faster and maintain speed more easily, especially uphill or under load.
Compared to Larger Displacement Off-Road Bikes (e.g., 650cc or adventure bikes): Larger bikes often have more outright power but can be heavier and less agile. A 450 dirt bike, especially a motocross or enduro version, will often be nimbler and quicker to accelerate in tight conditions. A true adventure bike might have a higher sustained top speed on the highway due to better aerodynamics and engine characteristics suited for long-distance travel, but it won't match the explosive performance of a 450 on its intended terrain.
Compared to Street Motorcycles: This is where the comparison becomes most stark. Even a modest 600cc sportbike will easily outperform a stock Honda 450 in terms of top speed, often reaching well over 100 mph. This is due to their more powerful engines, aerodynamic fairings, and gearing optimized for high-speed road use.

So, How Fast Can a Honda 450 Go? A Realistic Summary

To circle back to the initial question, “How fast can a Honda 450 go?”, the answer is nuanced. A stock Honda 450cc dirt bike, depending on the specific model and its intended use, typically has a top speed ranging from approximately 60 mph for a very trail-oriented setup with lower gearing, up to potentially 80-90 mph for a dual-sport model with taller gearing on a flat, paved surface.

If we're talking about highly modified bikes, particularly those converted to supermoto specification with street tires and optimized gearing, speeds could potentially push into the 90-100 mph range or even slightly higher, though this is venturing beyond the bike's original design parameters and requires significant rider skill and a safe environment.

It's crucial to remember that these figures are theoretical maximums in ideal conditions. In real-world riding, factors like terrain, rider ability, and environmental conditions will always influence the actual speed achieved. For many riders, the thrill of a Honda 450 isn't just about its absolute top speed, but its incredible acceleration, its agility, and its ability to tackle challenging terrain with confidence.

Frequently Asked Questions about Honda 450 Speed

How does riding a Honda 450 on pavement affect its top speed compared to off-road?

Riding a Honda 450 on pavement generally allows it to achieve a higher top speed compared to off-road conditions. This is primarily due to several factors:

Surface Grip: Paved surfaces offer significantly more consistent and higher traction than dirt, gravel, or sand. This allows the tires to grip better and put the engine's power down more effectively without excessive wheel spin.
Reduced Rolling Resistance: Knobby off-road tires have aggressive tread patterns that create more rolling resistance on hard surfaces than street-oriented tires. This resistance acts like a brake, limiting the bike's ability to reach its maximum velocity.
Terrain Consistency: Paved roads are typically flatter and smoother than off-road trails, which are often filled with bumps, berms, ruts, and obstacles. A smoother surface allows the bike to maintain momentum and reach higher speeds without being constantly interrupted by terrain changes.
Gearing: While stock gearing on many 450cc dirt bikes is optimized for off-road performance (favoring acceleration), dual-sport models (like the CRF450RL) are often geared taller to accommodate highway speeds. Even with stock gearing, the improved traction and reduced resistance on pavement allow the engine to push the bike closer to its theoretical top speed compared to when it's fighting for grip on loose surfaces.

Therefore, a Honda 450, especially a dual-sport variant, will likely achieve its highest speeds when ridden on a smooth, paved road, assuming it's geared appropriately and the rider is in a stable, aerodynamic position. This is why supermoto conversions, which utilize street tires and often adjusted gearing on a 450cc platform, can achieve impressive speeds on asphalt circuits.

What modifications are most effective for increasing the top speed of a Honda 450?

When looking to increase the top speed of a Honda 450, certain modifications have a more significant impact than others. It’s a balance of engine power, gearing, and aerodynamic efficiency. Here are the most effective modifications:

Gearing Changes: This is often the most straightforward and cost-effective way to alter top speed. To increase top speed, you would typically install a smaller rear sprocket or a larger front sprocket. This allows the rear wheel to spin faster relative to engine speed, enabling higher velocity before hitting the rev limiter. However, this will come at the cost of lower-end acceleration and climbing ability.
Exhaust System and Engine Tuning: A performance exhaust system, often paired with an aftermarket air filter and an ECU reflash or fuel controller, can significantly increase horsepower and torque, especially in the mid-to-high RPM range. More power means the engine can overcome air resistance and rolling resistance more effectively at higher speeds, thus pushing the top speed higher. Tuning ensures the engine runs optimally with these new components.
Cylinder Head Porting and Camshaft Upgrades: For more serious performance gains, having the cylinder head ported and polished, along with installing performance camshafts, can optimize airflow and valve timing to allow the engine to breathe and rev more freely, leading to higher peak horsepower and a potential increase in top speed.
Aerodynamic Improvements (Less Common on Dirt Bikes): While not typical for traditional dirt bikes, if the goal is maximum speed on pavement, even minor aerodynamic enhancements can help. This might involve adding a small windscreen or ensuring the rider is in the most tucked position possible. For bikes intended for more speed work, like supermoto, the smooth bodywork and rider position are inherently more aerodynamic than a standard dirt bike.
Tire Selection: Switching to a tire with a smoother tread pattern and a compound designed for pavement will reduce rolling resistance and increase grip, both contributing to higher potential top speeds on asphalt.

It’s important to remember that significantly increasing top speed often comes with trade-offs. Gearing changes will reduce acceleration, and radical engine modifications might reduce engine longevity if not properly maintained or if the bike is consistently ridden at its absolute limit.

Is it safe to ride a Honda 450 at its maximum speed?

Riding any motorcycle at its absolute maximum speed carries inherent risks, and this is especially true for a Honda 450, which is primarily designed for off-road performance. Safety considerations are paramount:

Stability: Off-road motorcycles, particularly motocross and enduro models, are not designed for high-speed stability on pavement. Their suspension geometry, tire choice, and lack of aerodynamic fairings mean they are more susceptible to wobbles, instability, and being affected by wind gusts or imperfections in the road surface at high speeds.
Braking: While Honda 450s have capable braking systems for their intended use, stopping distances increase dramatically at higher speeds. The brakes may also be more prone to overheating if used continuously at maximum velocity, especially on downhill sections.
Tire Limitations: Off-road tires are not designed for the heat and forces generated by sustained high-speed riding on pavement. They can wear out very quickly, lose grip, and potentially fail catastrophically.
Rider Fatigue and Control: Maintaining control of a powerful, lightweight motorcycle at high speeds requires immense rider skill, concentration, and physical stamina. Wind buffeting, vibration, and the sheer effort to stay stable can lead to fatigue, increasing the risk of errors.
Environmental Factors: Road conditions are unpredictable. Hitting a pothole, debris, or encountering unexpected traffic at maximum speed can have severe consequences.
Engine Stress: While Honda engines are robust, running them at their absolute redline for extended periods can place significant stress on internal components, potentially leading to premature wear or failure.

For these reasons, it is generally not recommended or safe to ride a stock Honda 450 dirt bike at its maximum achievable speed, especially on public roads. If seeking higher speeds, it's advisable to use a motorcycle designed for that purpose (e.g., a sportbike or a dedicated supermoto) and to do so in a controlled environment like a racetrack with appropriate safety gear and training.

How does the weight of a Honda 450 affect its top speed?

The weight of a Honda 450 plays a role in its top speed, though perhaps not as directly as engine power or gearing. Here's how:

Power-to-Weight Ratio: Top speed is a function of the engine's ability to overcome the forces resisting motion – primarily air resistance and friction. A lower weight means that for a given amount of engine power, the motorcycle has a better power-to-weight ratio. This allows it to accelerate more quickly and potentially reach a higher top speed because less of the engine's power is used simply to propel the mass of the bike and rider. Honda 450s, especially motocross and enduro models, are generally quite lightweight, which is a significant advantage for their performance characteristics.
Inertia: Lighter objects have less inertia, meaning they require less force to change their state of motion. A lighter bike can be accelerated and decelerated more easily, which is beneficial in dynamic riding situations. While inertia primarily affects acceleration and deceleration, it indirectly influences how quickly a bike can reach and maintain its top speed in varied conditions.
Handling and Stability: A lighter bike can be more agile and easier to maneuver. However, at very high speeds, a lighter bike might also be more susceptible to being unsettled by bumps or wind. This is why some larger, heavier motorcycles can feel more stable at extreme highway speeds, even if they have a lower power-to-weight ratio. The rider's ability to manage the bike's stability at speed is crucial.

In summary, the relatively light weight of a Honda 450 is a significant contributor to its lively performance and quick acceleration. While it might not translate to the absolute highest top speed compared to a much heavier, more powerful street bike with aerodynamic fairings, the lightweight design is fundamental to how a Honda 450 performs within its intended operational envelope.

Can a Honda 450 be made street legal and what impact does that have on speed?

Yes, it is possible to make many Honda 450cc dirt bikes street legal, though the process varies by state and can range from straightforward to quite complex. Honda itself produces dual-sport models, like the CRF450RL, which come from the factory with all the necessary equipment (lights, mirrors, horn, emissions controls) to be legally ridden on public roads. For converting a pure off-road model (like a CRF450R or CRF450X) to street legal, you typically need to:

Install a Lighting Kit: This includes a headlight (often with high/low beam), taillight with a brake light, and turn signals.
Add a Horn: A functioning horn is a legal requirement in most places.
Install Mirrors: Rearview mirrors are mandatory.
Obtain a Title and License Plate: This often involves a state inspection to verify the modifications and ensure the bike meets safety standards. You'll need to get a VIN inspection and apply for a title and registration.
Address Emissions: Some states have strict emissions requirements that can be challenging to meet on a pure dirt bike.

The impact of making a Honda 450 street legal on its speed depends largely on the specific modifications made and the bike's original design:

Dual-Sport Models (e.g., CRF450RL): These are designed with street use in mind. They typically have taller gearing than their motocross counterparts, which allows for higher top speeds on pavement. They also have more robust electrical systems to power lights and accessories. Their top speed is generally higher than a stock CRF450R, potentially reaching into the 80s or low 90s mph range.
Converted Off-Road Models: If you convert a CRF450R or CRF450X, the top speed will be influenced by the gearing you choose. If you opt for taller street gearing to achieve higher speeds, you will sacrifice off-road performance. The addition of street-legal components themselves (lights, etc.) usually doesn't add significant weight or drag to affect top speed drastically. However, the rider's posture for road riding (less tucked) and the limitations of off-road tires on pavement will still be limiting factors.

Essentially, transforming a dirt bike into a street-legal machine often involves compromises. While you gain the ability to ride on public roads, achieving high sustained speeds might require specific gearing changes, and the inherent design of a dirt bike means it will likely never be as comfortable or as fast on the highway as a motorcycle purpose-built for that environment.