what is the formula

what is the equation that governs the relationship between the speed of an automobile moving on the road and the speed of its prime mover?

now to define my problem more clearly, here is an example:

an automobile has to be designed such that , it has to be powered by an electric motor

the following values have been specified as data:

the maximum road speed (km/h)
maximum payload (kg)
mass of the entire vehicle (kg)
wheel diameter (m)
the vehicle is single speed and motor is coupled to the rear axle by bevel gears,

what we have to determine:

max torque of the motor
horse power of the motor
speed of the motor (rpm)

i already have the formula to determine the torque. what is the formula to determine the required horse power and speed of motor provided we assume any values of the number of bevel gear tooth