For anyone is interested and that has a Google account, I've created a spreadsheet to calculate the required motor RPMs for a set of inputs.
It takes wheel diameter in inches, miles per hour, and gear reduction ratio and then calculates the RPMs at the motor shaft required to get that. Wheel diameter and miles per hour are for the prototype size. The gear reduction ratio would be for the model, and is the total reduction from the motor output shaft to the axle gear.
https://docs.google.com/spread...nME/edit?usp=sharing
P.S. the green boxes are what are user editable.