Re: theoretical top speed?
Ok the math is spot on... but 8:1 gearing is going to require a lot of torque to be able to accelerate to reach those speeds, not to mention wind resistance which could prevent it from reaching the speed you're looking for...
What are you using for an engine? 4 stroke or 2 stroke? and is it set up as a single speed all the way or does said engine have an internal transmission like a small motorcycle?
My guess is you'll need at least 7 or 8 horsepower to reach these speeds, not to mention it will need a lot of low end torque to be able to accelerate well enough for those speeds to be achievable within a reasonable distance...
It is a good ratio for a small 7 to 12hp diesel engine tho if you got the frame space and the frame is strong enough... if you think about it, small diesel engines produce a LOT of torque for their horsepower and are about the same size as a comparable gas engine, but heavier. Typically a 7hp diesel will feel about the same as a 12 hp gas engine due to the high torque output. the only drawback is that they only like to rev to about 3500 to 4000 rpm but can be ungoverned to get maybe another 1000 rpm out of them, but with 8:1 final gearing it would still be plenty fast...