it would help to have a test capx file to look at, otherwise you are only going to get vague suggestions...
but what you want to do is "scale" the speed to a range of playback rates.
so, in the Set Playback Rate action, you would set Playback rate to:
1.0 * car.Platform.Speed / car.Platform.MaxSpeed
obviously, that is assuming you are using the Platform behavior. Also, you would have to find what range of rates sound good with the sound effect you are using - what would be the minimum rate and what would be the maximum.
if the minimum rate you want is 0.5 and the maximum is 1.4, then you would need to set the rate to:
0.5 + 0.9 * Sprite2.Platform.Speed / Sprite2.Platform.MaxSpeed
or as a general function: set playback rate to: min rate + (max rate - min rate) x current car speed / max car speed
you might want to adjust the volume of the engine sound by scaling it in a similar way...