It wouldnt be too hard to get them to go 40mph. You'd have to gear the car very high, have the car tuned to death, and one hell of a flat, grippy straight to get speeds upto and past 50mph.
Onroad circuit cars can reach 60mph+ on a short flat straight easy, as they are built for acceleration and top speed.
ok kl i might look at that sometime and it was the same for me when i was living in france ther was 1 model club with a buggy/truggy track near me but i dont think ther was another 1 within 100 km