Hey flick, I meant to post up for that thread, but forgot. Speed is actually directly proportional to power not revs. My dremel does 35,000 revs but it ain't gonna propel me nowhere. Basically an engine must overcome every opposing force to move you. These can all be measured as resistance in lb's. Rolling resistance remains relatively constant over all speeds and comprises weight of rider and machine, the road surface, tyre inflation etc. But the biggy is air resistance and drag. It increases greatly as the speed increases. There is a formula for calculating resistance. The frontal area of rider and machine in square feet x the velocity squared x the drag + the rolling resistance. This will give you the horsepower required for any given speed. Gearing is negligible except for keeping the engine within its greatest power producing range or if you still have power but run out of revs.
For example, some guys in the states are making 90 mph with heavily worked motors and are trying to hit 100. But what is standing between them and 100 mph is about 5 hp.