I threw together a quick tach example this morning to see what kind of troubles you might be having but it was pretty straight forward to me.

Set your animation to 30fps (real time). Drag your revving engine wav file onto the timeline at frame 1. You should now see the waveform and be able to scrub back and forth. Now just keyframe the tachometer's needle to the changes in the sound. I think with something like this, you'd be better off animating to sound rather than trying to fit the sound to your animation.

I'm sure there is more than one way to achieve what you're trying to do (if you need another framerate or whatnot) but it's all right there in the helpfiles.