I am working on a vehicle. For this, I have to design a gear box which is to have some reduction ratio. Now the problem is I don't know how to do that. I know my engine specifications ( for example : Max net power: 5.38 KW (7.2 PS) @ 7500 rpm Max net torque: 7.66 Nm @ 5500 rpm) According to my calculations, total opposing force acting on vehicle is 344N and radius of rear wheel is 0.356m. Going by that, torque required at rear wheel is 122Nm. Now if we divide these two to torques (rear wheel to the engine), we get a reduction ratio of 16:1. But The reduction ratio in normal vehicle is less than equal to 10:1. that is why I am not convinced about my way of calculating reduction ratio. My vehicle is a 3 wheeler with total height of less than 0.6 meters and a total length of 3 meter. So Is reduction ratio affected by length of vehicle?

hi may be your vehicle isn't a normal one! you must not limit your mind because of that. i think you need a kind of test or simulation. why do you think reduction ratio is affected by length of vehicle?is it because of drag force?

I don't believe vehicle length matters in this respect. But motor speed does. How fast is your motor intended to run? (I mean practical max and min.) AND How fast do you want to drive? That is the ratio that should steer your gearbox choice: motor angular velocity to wheel angular velocity. If the power is not enough, then you can choose between going for greater reduction ratios in your gear box and driving veeeery slowly, or upgrading your motor and sticking to a sensible gear box, if you still want to drive at all. If the motor is supposed to run at constant speed, and considering the moderate power, the best solution is a continuous drive (most probably a variable diameter belt drive). But this does not seem to be your case.