In rFactor, you have to set the engine sound samples to match correct rpm's manually, by editing a text file. I created a little tool that would help to tune the values in real time, so you don't have to reload the game over and over again after each small change. Works with rF1 and rF2 style SFX files.
It is currently in still in beta stage, many features are missing and bugs need to be sorted out. Basically, what you can do right now, is specify the sound base dir (for example <rFactor folder>/GameData/Sounds) and then open an SFX file of a car. You will be displayed a tachometer and a list of sound samples. You can rev the engine (either automatically or manually) and change the min/max/natural values of the samples. You can also save the result.
UPDATE 18-11-2012: Version 0.3 (and source code!) - Added missing 'save' functionality.
Download binaries: EngineSoundEditor-bin-v0.3
Download source: EngineSoundEditor-src