I guess that you are really young?

50 mph is not equal to 100 km/h. But anyhow this rtr will not go that fast unless towed behind a real car.

The speed of any buggy only depends upon the gearing and the rpm of the engine. If you have a high topspeed then you have slow acceleration and in a buggy you want acceleration. The topspeed are irrelevant.

You should do some research.