re-reading and noting that this is in friction drive makes me refute my earlier statement...
hmms. yes, the engine can run direct in friction drive. you have to consider the diameter of the roller only. think of it as the wheel running along the ground. the tyre itself, well...its speed is exactly the same as the ground its rolling along. so we can ignore tyre diameter for friction drive setups.
get the roller diameter right and the gearing will be right. basically, around 1" diameter is good.... smaller for hills and power, bigger for speed.
or pi x 1" = 3.14 inches of travel per revolution of the engine. times that by rpm and remove the necessary zeroes to get speed in mph, kmh, kn, parsec...iunno...
hint... 1/2" external star washers make excellent rollers 7/16 or larger, i never tried.