How Fast is a 125cc Dirt Bike | Dirt Bikes Action

If you are looking for a speed upgrade of 50 cc, 85 cc, and 100 cc dirt bikes, you should take your eyes on 125 cc dirt bikes. With the addition of more CCs, the engine now becomes better, faster, and more powerful too. But, if you are concerned about How fast is a 125cc dirt bike, let’s find out this part in today’s speed guide.

A 125 cc dirt bike is a common model dating back to the 70s and 80s due to its tremendous power, lightweight body, optimal control for teens, and budget price (of course) too. There is a major debate on who built the first 125 cc. Whatever brand it is, the outcome is a spread of 125 cc dirt bikes across the nation and the world as well.

Nowadays, this is one of the most common engine sizes you will find among dirt bikers. Speed ninjas (I as well) are always looking to get into speedy things and for this current model, they’ve been found curious about its speed limit. How fast is a 125cc dirt bike? Really depends on its make, model, engine type, terrain, and rider experience.

However, the nicest 125 cc dirt bike out there is supposed to run at 75 mph top speed (the one by KTM with a 2 stroke engine). The least you can expect a 125cc is to be at 40 mph, a 4 stroke model by Yamaha.

Let’s find out every single thing affecting the potential top speed of a 125 cc dirt bike!

Factors that Determine the Speed of a 125cc Dirt Bike

125 CC is a standard size engine all around the world. But, you will see a variety of changes in horsepower, loading capacity, top speed, mileage, and a few other things. Why is there a difference despite being the same in size?

The answer is; there are many variables reacting to a 125 cc engine resulting in achieving the highest or potentially lowest speed of it. We can’t list everyone out of those, but these are common things:

• Advertised VS Actual Max Speed

First thing first, most (or almost all) dirt bike manufacturing brands advertise the highest top speed that you will never achieve unless by luck. For example, a particular brand advertises its 125 cc dirt bike machine with a top speed of 85 cc. But, you won’t get this much speed.

Because they test their model after manufacturing in different circumstances and with several kinds of parameters or methods like the one BMW does. This doesn’t mean you will achieve that too, but this shows the vehicle had touched this top speed during that “S” test.

Another reason, a few makers may limit their bikes to a certain speed for safety concerns, both for riders and the machine itself. So, if a bike is advertised at 80 mph for its top speed, expect it to do 75 mph at max. And that too with recommended safety and care practices like the quality of fuel, the thickness of tires, load, and riding dynamics.

Besides these safety and technical aspects, another thing that matters is the pricing of your machine. If you look at a budget bike, the top speed will surely not be high. And, if a nice and expensive bike is to be taken, you can expect its top speed to be comparatively better and faster. An $800 125 cc bike will not hit 50 mph but a $1000 bike will definitely do.

• Four Stroke vs Two Stroke

Not just the brand, but the type of engine too affects the potential of maximizing your bike’s top speed. Design wise, all the engines may look similar and you can’t easily notice the difference when looking at them. However, you will see a considerable speed difference in both of these.

Because they have a different type of engine installed in them. Design and working wise, there are two kinds of petrol engines. A 2 stroke engine, has a power cycle of just two strokes. The piston in the crankshaft takes 2 full rounds to complete that cycle covering injection, ignition, combustion, or expansion, and then exhaust. All that works in just 2 cycles. So, work is done faster making the bike faster.

A two stroke 125 cc engine may be able to produce even 100 mph but 70 mph is quite common with these engines. The least you can expect is around 50 to 55 mph officially, without any kind of alteration. So, you will enjoy the perks of having a 2 stroke engine in your 125cc dirt bike.

Four strokes, on the other hand, are definitely slower than a 2 stroke. The work cycle still consists of 4 cycles and each work takes a complete cycle to get done. So, a 4 stroke engine completes its work cycle in 4 cycles including injection, ignition, expansion, and exhaust.

• The Engine Size

Yes, Size matters! (Oh Man, we are talking about the size of an engine). The size of your engine is directly correlated with the overall speed and performance of your overall dirt bike. And, engine size is measured in CC, a short form for engine size in cubic centimeters. The bigger the size, the bigger the engine and the more miles per hour it will add to the meter. 

• The Gear Ratio

To a major extent, the gears of your bike can definitely affect your top speed. Each bike has gears (see this guide on the pattern of dirt bike gears) and every gear is supposed to be limited to providing a certain speed. For example, you can’t get your bike beyond 20 miles per hour in the first gear. Trying it otherwise will just move the RPM needle but not the speed. Similarly, the fourth gear is there to touch 80 mph speed!

Dirt bikers may move around non asphalt and uneven roads with different shapes and lengths of patches. If you are out there on a short patch of under a couple of hundred meters, limit your bike to a maximum of 3 gears being engaged. And, if you are on a long patch, you should be beyond the 3rd gear, be it 4th, 5th, or 6th gear if your bike has got it!

This common sense may not only save you gas and time, but it will also get the best power and speed out of your bike when it’s in the right gear at the right time. In another way, not just these gears but your chain and sprockets will also affect the potential top speed of your dirt bike.

You set up the gear combination in a way where the large gear with more teeth should be at the back and the smaller one with low teeth in front of it. This is known as the base gear ratio and it should always be set in this way. So, you can ensure the best effective transmission of power from the gear to the rear wheel with the least possible loss. Dude, this is a simple hack you might not know. Do this right now and thank us later.

• The Modification

What kind of aftermarket products and accessories, or what type of gear you have added up on your bike may also affect the top speed. That is due to two main reasons: due to suboptimal build quality and also due to mismatch or just putting it improperly. To counter that, the only way is to throttle your bike and rearrange the gear and sprocket.

• Load & Ride Environment

Well, this is an external factor but it can never be neglected at all. The terrain you ride at, the way you’re driving, and the total load you have put on your bike, all these things surely matter in speeding your bike. Terrain specifically affects your speed and we are not talking about asphalt or carpeted roads as this is not the official pitch of a dirt bike.

On rocky terrain, the speed will be different, you can’t even touch the other half of your meter if you are in mud or gravel. On any kind of downhill or climbing a cliff, you can’t and should not ride at a top speed. And a lot of other things that may directly affect your overall speed. Similarly, the total load the bike is bearing will not let it do a full meter.

• Experience of Rider

Yeah, this is undoubtedly a significant factor affecting the speed of a dirt bike. As you have been asking about how fast a 125 cc dirt bike is, it really depends on the way you drive and the experience you have got under your belt. All things collectively affect the top speed.

If you don’t have ample experience and you try to hit the top speed, you are just risking your life as you are prone to lose control sooner. Unless you have mature hands and a healthy grip, you should never push it beyond its limit. Else, you will be injured or wasted. Better avoid it!

There are many bike brands and several 125 cc models under them. The best 125 cc dirt bikes are supposed to hit their top speed in under 20 to 35 seconds if your way is clear and everything is under your control. The bigger your engine gets, the less it would take to hit its top speed.

• Maintenance Practices

Last but not least, if you want to keep hitting your bike’s top speed without being problematic you should start taking care of its maintenance and care from day one. Believe me, it’s definitely worth it. Also don’t Overspeed, or else you will notice frequent wear and tear in it.

Start with adjusting its suspension and fixing its exhaust. These two things are the least considered but it matters a lot. If you are having a stiff suspension, the weight distribution will not be perfect and it may also lead to compromised control and rideability.

Also, if you’ve installed aftermarket tires and they are bulky, it may provide a better grip on certain terrains but will also reduce the speed. With lighter tires and proper thickness, you get both speed and grip. Not just that, it will also improve your credibility with a top speed.

Best 125 cc Dirt Bikes Speed Comparison

With the top dirt bikes moving in the market, you should take notes of their top speed and can also compare them with your existing one to find the difference that you can improve by shifting to the best 125 cc dirt bike. Take a skimmed look at this given speed chart by bike brand with their engine capacity, type of engine, and top speeds in miles and kilometers.

The given are facts and figures that can vary from actual top speeds*

125 cc Dirt Bike Speed Frequently Asked Questions

• How Fast Can a 125 cc dirt bike go?

The best 2 stroke dirt bike out there in the market can go as fast as 75 miles per hour which are around 120 km per hour. However, some experts have seen even 100mph which is obviously not a safe thing to do. Anyways, you can expect an average of 125 cc to be as fast as 60 mph being the top speed. Yamaha’s, however, is limited to just 45 miles per hour.

• What age is a 125 cc dirt bike for?

Anyone with skills and the ability to drive a dirt bike can ride a 125 cc too. For the age thing, there is no certain age limit for a bike of this size. From a kid to a teen and an adult, anyone can ride a 125 cc dirt bike. However, from a safety perspective, it is not for kids under 12 years of age. If your kid has started to improve his skills, you can get one for him!

• What is the height of a 125 cc dirt bike?

For a fun, loving, and adventurous dirt bike riding experience, you should consider looking at its seat height as well. So, you can ensure that it’s the right size as per your height. If you are under 5 feet and 6 inches, you should get a dirt bike with a seat height of 30 to 33 inches. Anyone shorter than 5 ‘6” should go for under 30 inches seat height and 5 ‘7” and more height, should go for a 34 and 36 inches and plus size.

• How much horsepower does a 125 cc have?

A 125 cc dirt bike may have a horsepower of at least 30 to a maximum of 38 horsepower. The horsepower of a 125cc dirt bike engine may vary from brand to brand but it directly depends on the torque and RPM of an engine. These RPMs may vary from 4500 to 15,000 per minute even.