Yeah, I understand that. Just a key is not enough to control the power of the Roadster. A gaming pedal or the shoulder/trigger of a gamepad is necessary for slower acceleration, or you could simulate the pedal with a throttle bar. For example: W = increase throttle, S = decrease throttle, nothing = throttle keeps fixed, Z = max throttle input, X = zero throttle input.
But what I'm talking about can be seen in the other cars too. You can see it very easily this way: Get a very high speed and jump over a ramp, when the car touches down, it will accelerate again for a while like if you were still pressing the W. Did you make sure the RPM are exactly what they should be?