In stock trim, a Honda TRX400EX typically tops out around 60–65 mph (97–105 km/h). Some riders report up to about 70 mph under optimal conditions.
The Honda TRX400EX is a two-wheel-drive, sport ATV produced in the late 1990s and 2000s. It uses a 397cc air-cooled four-stroke single and a five-speed transmission with manual shift and electric start. Because top speed depends on multiple factors—model year, gearing, tire size, rider weight, terrain, and maintenance—real-world results vary widely. The vehicle’s speed is effectively controlled by final-drive gearing and engine power rather than a dashboard limiter, so modest changes can shift performance noticeably.
Stock top speed expectations
These figures reflect common stock configurations on flat terrain with standard tires and typical riders. Actual top speeds will vary with the conditions described below.
- Typical stock top speed on flat ground: about 60–65 mph (97–105 km/h).
- Under favorable conditions and light rider weight, some riders report top speeds near 70 mph (112 km/h).
- Rider weight, terrain (hills, sand, mud), altitude, and ambient temperature can push the observed top speed up or down by several mph.
- Model-year differences are relatively small; most 1999–2008 TRX400EX versions share similar gearing and power characteristics.
These numbers provide a general sense of what to expect from a stock 400EX; riders should treat them as approximate guidelines rather than guaranteed figures.
Ways to potentially increase top speed
To understand how riders attempt to push the 400EX beyond stock performance, several modification paths are commonly pursued. Each has trade-offs in acceleration, reliability, and handling.
Gearing changes
Adjusting the final-drive ratio is a common way to raise top speed. Changes can affect launch and hill-climbing ability, so proceed with caution.
- Increase rear sprocket size to raise top speed by a few mph (and reduce low-end torque).
- Or, reduce the front sprocket size to shorten gearing and tailor to higher speeds, with similar trade-offs.
- Account for chain wear and sprocket compatibility; longer-term reliability depends on maintenance.
Gearing changes can yield noticeable top-speed gains, but expect slower acceleration and potentially greater stress on the drivetrain.
Tire and weight adjustments
Both tire diameter and rider/gear weight influence speed and responsiveness. Lighter setups and larger-tiameter tires can shift the top-end, but with caveats.
- Install slightly larger-diameter tires to gain a higher final speed reading, at the cost of acceleration and handling accuracy.
- Remove unnecessary weight (accessories, aftermarket plastics) to improve power-to-weight ratio.
- Keep in mind that larger tires can affect speedometer readings and overall handling.
Weight and tire choices offer a straightforward route to a higher top speed, but they balance against torque and handling.
Engine and intake/exhaust
Engine breathing and exhaust flow can free up horsepower, which helps sustain higher speeds.
- Exhaust upgrades (free-flow) and intake improvements can yield modest horsepower gains.
- High-compression pistons or cam upgrades can increase power, but often require tuning and may affect reliability and heat management.
- Carburetor calibrations or rejetting may be necessary after intake/exhaust changes to maintain safe air-fuel ratios.
Engine and intake/exhaust modifications can meaningfully affect top speed but demand careful tuning and professional installation to preserve reliability and safety.
Summary
The Honda 400EX generally reaches about 60–65 mph in stock condition, with some riders reporting up to 70 mph under optimal conditions. Top speed varies with model year, gearing, tires, rider weight, terrain, and maintenance. For those seeking higher speeds, gearing adjustments, tire choices, weight reduction, and engine modifications are common routes, each with trade-offs in acceleration, durability, and safety. Always ride within your skill level and local regulations, and use proper protective gear.