DC Motor Exact Speed Control with 8051

Hi,
Although there are a lot of similar questions with almost the same titles in the forum, my question is bit different than them. Firstly, I am going to use L293d and two 12V DC, 200rpm motors to construct the circuit. And I am going to use the PWM method to change the speed of these motors. I intend to do a small project in which I make a car with two motors and a IR distance sensor. (8052 (s52), 12-bit ADC, L293d etc.) First, there will be a test drive where I am going to measure the distance and the elapsed time in which the car reaches a designated location with 100% pwm. Then, I will ask for input from the user which will determine the time the car reaches the same distance. For example, let's say the car completes his track with maximum pwm in 5 seconds. Would it be true to assume that if I want this car to reach the same distance in 10 seconds, then I have to change pwm to 50%(half of the maximum power), given that the load on the car doesn't change? Or are there some other calculations to determine the exact speed of this car? In other words, is rpm of a DC motor proportional to the pulse-width modulation?
Thanks for all the answers in advance!

hi,
u have answered ur question already? just change pwm to 50%.

Speed of the motor is directly proportional to the voltage applied. Voltage applied is directly proportional to Pulse width.
I think that might clear your doubt
Thanks