So something like this:

The fast response of a petrol engine that's sounds very sharp and very aggressive:



The bit slower response of a diesel engine that sounds very heavy and does more the lower raw sound.



And same with the revs!

It would be cool if you had 2 acceleration buttons too. Like 1 for full power till the end of power and revs and a acceleration button for 50% to 75% of the power (if you want to drive more slow and precise). I remember driver 1 and Driver 2 where you had these buttons.