Re: 24 m.p.h.???
in my experience, it's sprocket size, not the tire size.
my 24" with a 28t sprocket out accelerates any 26 i've come up against, and has a higher top speed.
a stock 26" i built with a 40t sprocket does 31 according to the school speed traps.
in fact, my 24's faster than all the 26's i've built, too (so far.)
the way to make these bikes faster (top speed) is to tune the motor to get it's peak performance, then gear it as low as you can where it'll still pull you around.
my theory is, you could put a 16t on the rear engine side and a 60t on the pedal side. once you pedaled the bike up to speed the engine will take over until it hit's it's max rpm.
you become the transmission, and the engine becomes the final drive.
that's my bonneville speed theory anyway...