measure the muffler noise of your car; in other words, a decibel meter. After all, how loud is too loud? Maybe you can borrow or rent one from a stereo shop or PA system (rock music type) rental place. Science teacher?
Measure and document (with photos) the sound level of your car at various rpms. Here's the hard part: get a disinterested, third party witness to attest to the measurements in writing. Have their written statement signed by them with their signature notarized. All together, that's some pretty good evidence the officer is mistaken.