Unfortunately not very fast. My brother inlaw is a diesel mechanic and he adjusted it to have a little more power and wants to do new fuel injectors. With the original injectors we can maintain 55mph on a flat road. However, small hills slow us down to around 45mph and really steep/long hills down to 35mph at the worst. Then going downhill we get up to 65mph-70mph. I didn't buy a bus to be in a hurry but that certainly was disappointing...hoping to replace the injectors this fall. We'll see how much of a difference that makes.