The usual assumption is that 1 quant= 1 inch= 2.54 cm

In the car demo there's a linear relationship between how much gas you apply and the velocity that you get. I also added a maximum velocity that can not be exceeded. For a manual transmission you would have to implement a maximum velocity for each gear. E.g. the second gear only allows velocities up to 30 kmh. Of course it also affects the amount of wheel torque available for accelerating. In the demo this wheel torque is a constant and you would have to adjust the script to take into account what gear you are in (low gear= high torque, high gear=low torque).