A small script for Blender Game Engine Python to change the pitch of the car engine sound depending on the speed of the car.

EngineSoundScript.py

def engineSound ():
    from bge import logic
    
    cont = logic.getCurrentController()
    
    # Name of the Sound Actuator playing your sound
    sound = cont.owner.actuators["enginesound"]
    
    # Replace "Car" with the Name of your car
    speed = abs(round(sum(scn.objects["Car"].linearVelocity), 2) * 0.25)
    
    cont.owner["speed"] = speed
    
    # Kind of "Gears", defines the speed after which the rounds-per-minute of the engine "reset"
    if speed < 2:
        multiplier = 0.15
#    elif speed < 3:
#        multiplier = 0.1
    elif speed < 6:
        multiplier = 0.05
    elif speed < 9:
        multiplier = 0.035
    else:
        multiplier = 0.02
    
    # Again - replace "Car"
    pitch = abs(round(sum(scn.objects["Car"].linearVelocity), 2) * multiplier)
    
    if pitch > 1.25:
        pitch = 1.25
    elif pitch < 0.5:
        pitch = 0.5
    
    # Prints the pitch to the system console. Not noecessary, just for debugging
    print(pitch)
    
    sound.pitch = pitch
    
    sound.startSound()