How Fast Does a Honda 400EX Go (mph)?

The Honda 400EX (officially TRX400EX, later TRX400X) typically tops out around 65 mph in stock condition, with most well-maintained machines registering a realistic 62–68 mph on level ground; light modifications or ideal conditions can nudge it into the low 70s.

What the 400EX Is and Why It Matters

Honda’s TRX400EX is a legendary sport ATV known for its durable air-cooled 397cc single-cylinder engine, five-speed manual gearbox with reverse (on later models), and balanced chassis. Sold primarily from 1999 through the late 2000s (with a later refresh as the TRX400X), it has become a benchmark for reliable trail and dune performance. Top speed is a frequent question because it indicates both the engine’s character and how the machine stacks up in the sport-ATV class.

Real-World Top Speed

In independent rider tests using GPS and radar on flat terrain, stock 400EX quads typically reach the mid-60s mph. Variability comes from rider weight, tire size and pressure, altitude, surface (hardpack vs. sand), wind, and the stock rev limiter, which cuts ignition in the low 9,000 rpm range. Later TRX400X models share essentially the same engine and show similar results.

Typical Numbers You’ll See

Owners commonly report 63–66 mph on trails and hardpack, about 65–68 mph on long, flat straights with proper jetting and tire setup, and up to about 70–72 mph only when aided by a tailwind, slight downhill, or mild modifications (such as gearing or ignition changes). Speeds above that generally require more extensive engine work.

What Affects the 400EX’s Top Speed

The following points outline the main factors that can raise or lower the 400EX’s peak mph in real use.

• Rider weight and position: Heavier riders or an upright posture increase drag and reduce top speed; a tucked position can add 1–2 mph.
• Surface and traction: Smooth hardpack yields higher speeds than sand, mud, or grass, which sap power.
• Tire size and pressure: Taller rear tires effectively raise gearing and can add top speed (at the expense of acceleration), while underinflation increases rolling resistance.
• Altitude and temperature: Thinner air at higher elevations and very hot conditions reduce engine output, trimming mph.
• Wind and aerodynamics: Headwinds slow the quad; tailwinds can push speeds a few mph higher.
• Engine health and tune: Clean air filter, correct jetting/fueling, fresh plug, and healthy compression are crucial to reach the mid-60s.
• Rev limiter and gearing: The stock CDI limits rpm; sprocket changes alter the speed at a given rpm, trading acceleration for top-end or vice versa.

Taken together, these variables explain why some stock 400EX units clock closer to 62 mph while others reliably hit 66–68 mph under identical instrumentation.

How to Reach or Slightly Exceed Those Speeds

If your machine is falling short or you’re seeking a modest bump, the following common adjustments and upgrades can help. Always verify fueling after intake or exhaust changes and ride within your limits.

• Maintenance and tune-up: Fresh oil, clean air filter, proper valve clearances, correct jetting, and optimal tire pressure often recover several mph on neglected machines.
• Gearing changes: A one-tooth larger front or a few teeth smaller rear sprocket raises top speed but softens low-end pull; ensure the engine can still pull top gear to redline.
• Intake/exhaust and jetting: A freer-flowing muffler and filter, paired with correct jetting or EFI tuning (where applicable), can add a few mph and stronger midrange.
• Ignition/CDI: An aftermarket CDI with a higher rev limit can increase indicated top speed if the engine has the power to pull the added rpm.
• Tires and rolling resistance: Properly inflated, round, and balanced tires reduce drag; a slightly taller rear tire can increase terminal speed.
• Engine work: Cams, high-compression pistons, porting, and big-bore kits can push well past stock limits, though reliability, heat, and cost trade-offs grow.

For most riders, a well-tuned stock engine with minor gearing and intake/exhaust tweaks is enough to see high-60s to low-70s mph in favorable conditions, without sacrificing the 400EX’s famed reliability.

Safety and Legal Considerations

High-speed ATV runs should be conducted on closed courses or private property with adequate runoff and proper safety gear, including a DOT-approved helmet, gloves, boots, and eye protection. Observe local laws and land-use rules, and remember that braking distance and stability change markedly above 60 mph on uneven terrain.

Bottom Line

Expect a stock Honda 400EX/TRX400X to reach roughly 65 mph, with a common real-world range of 62–68 mph. With ideal conditions or light modifications, nudging into the low 70s is possible, but sustained higher speeds typically demand deeper engine and gearing changes.

Summary

The Honda 400EX’s top speed is about 65 mph in stock trim, with real-world results spanning 62–68 mph depending on setup and conditions. Proper maintenance, small gearing tweaks, and intake/exhaust tuning can add a few mph, while extensive engine work is needed for significantly higher speeds. Ride safely and tune thoughtfully to balance performance with reliability.

How fast is a 2002 Honda 400EX?

The top speed of a 2002 Honda 400EX (TRX400EX) is generally cited as being around 65 mph to 72 mph, though this can vary based on specific conditions and modifications. Some sources give a rating of around 65 mph, while others, and actual rider tests, indicate speeds up to 72 mph have been reached. 

• Advertised Speed: Some sources, like webBikeWorld and Ron Bishop Motorcycles, list a top speed around the 72 mph mark. 
• Real-World Performance: Rider tests and videos show the 400EX can reach or exceed 65 mph. 
• Factors Affecting Speed: The actual top speed can be affected by factors such as rider weight, terrain, tire pressure, and whether the ATV is stock or has aftermarket modifications. 

How fast does a 400 quad go?

A 400cc ATV can typically reach speeds between 50 and 70 mph, with variations depending on factors like the specific model, terrain, and modifications. Many 400cc ATVs are designed for a balance of power and agility, making them suitable for both recreational trail riding and light utility tasks. 
This video shows a top speed test of a Suzuki LTZ 400, reaching 72 mph: 58sSouth Cac ChuckYouTube · Jul 26, 2024
Here’s a more detailed look:

• Engine Size and Top Speed: While 400cc is a general displacement, the actual top speed can vary. Some models like the Suzuki LTZ 400 might reach 70 mph. Other 400cc models, particularly those geared towards utility or sport-utility, might have slightly lower top speeds, closer to 50-60 mph. 
• Factors Affecting Speed:
  • Terrain: Smooth, flat surfaces allow for higher speeds compared to rough, uneven terrain. 
  • Modifications: Aftermarket parts, like tires or engine tuning, can increase or decrease speed. 
  • Rider Weight and Riding Style: Heavier riders and aggressive riding styles can impact speed. 
  • Maintenance: Proper maintenance, including engine tuning and tire pressure, is crucial for optimal performance. 
• Safety Considerations:
  • It’s crucial to ride within your skill level and adhere to safety guidelines. 
  • Always wear appropriate safety gear, including a helmet. 
  • Be aware of your surroundings and other riders. 
  • Familiarize yourself with local laws and regulations regarding ATV use. 

This video shows a speed test of a 1996 Yamaha Kodiak 400: 51sWe Try MotorsportsYouTube · Nov 16, 2020

What is the top speed of a 2004 Honda 400EX?

The 2004 Honda 400EX, also known as the Sportrax 400EX, has a top speed of 72 mph. This four-wheeler is powered by a 397cc engine and features a five-speed transmission. 
This video shows someone testing the top speed of their 400ex: 59sBrewer OffroadYouTube · May 2, 2020

How fast is the Honda 400 EX?

The Honda 400EX (TRX400EX) has a stock top speed of approximately 72 mph (116 km/h) on pavement, though this can vary slightly depending on rider reports and modifications. While some riders report speeds around 64-68 mph, others have documented reaching speeds closer to 70-72 mph, even on stock machines.
 
Factors Affecting Top Speed

• Pavement vs. Off-Road: The 72 mph figure was achieved on pavement, with the vehicle needing some coaxing to reach it. 
• Rider and Environment: Factors like the rider’s weight, terrain, and wind conditions can influence the actual top speed achieved. 
• Modifications: Performance upgrades, such as Elka shocks and ITP bead-locks, are common on these ATVs and can affect their speed capabilities. 
• Condition of the ATV: A stock, brand-new condition will yield the manufacturer’s reported top speed. 

What Riders Say 

• Many riders consider the 400EX to be a versatile and durable machine capable of excellent performance for challenging trails and leisure rides.
• The machine was designed for a balance of performance and reliability, making it a favored choice among ATV enthusiasts.