If I understand you correctly, you want to calculate the acceleration rate rather than speed or the force that pushes the car..
A tip, in a function,
set temp to the current speed of the car...
Then, in the next frame, set whatever variable you use as "acceleration speed" like this.
accell = (current_speed - temp) * time_step;
The goal is to compare how many units the mph/kph has increase in each from relative from the previous frame.
If frame 1 you're going 50 mph and in the next from you're going 55, you've accelerated at a rate of 5 mph per frame (which is actualy pretty fast!).
Another thing is you have to multiply the result by time_step or any difference in the framerate will render the results inaccurate.
It might be something like this.
////////////////////////////////////////////
var speedmph;
var accell_rate;
(c-script style example)
function calc_accell_speed()
{
while(1)
{
temp = speedmph; //OR YOUR speed variable
wait(1);
accell_rate = (speedmph - temp) * time_step;
//// acceleration is mph in current frame MINUS mph in the previous from - compensated by timestep.
}
}
///////////////////////////////////////////