The top speed for a YZ125 2-stroke is around 70mph, much faster than the 4-stroke machine which has a top speed of around 50. This of course depends on the model you buy as not all have the same top speed. The average speed of 70 mph comes from a wide range of different models. This can increase depending on the terrain you are riding on. Suppose you knock down a runway. You will go faster than when you drive on really rough terrain.