Im building a model car with mass 0.3kg. Im thinking of buying a fancy 6 speed gearbox. But before I spend my money, I need to know if any of the gear ratios are going to move my little car. Here is the motor: the gearbox has ratios: 11.6:1 29.8:1 76.5:1 196.7:1 505.9:1 1300.9:1 If I wanted maximum speed, but I want my car to run, which gear ratio should I use? I have four wheels, each 0.0028m in radius. Im using a 3V battery. The gearbox drives an axle which runs through two wheels. The torque applied by the axle is through the centre of rotation. The coefficient of rolling friction between the ground and the tire is about 0.01, maybe a bit less since the tires are so small. I know that F = umg where u is the coefficient of friction but thats it. Please teach me how to work out a similar problem for next time. I have a feeling that I will be building alot of model cars in the future.