Movimotors run around 3000rpm. Some a bit faster, others a bit slower.

Cart tyres are not small, but the difference between fronts and backs is substancial. What size did you have in mind?

Imagine a 10" wheel. 3000rpm/8= 375 rpm on the wheel. Gives around 17 kph.
Overvolt the motors to 36V, and the topspeed goes up to 25kph+

It's up to you, but I like higher voltages and bigger gearratios.