in that case, I say your cyclocomputer idea is fine. There's no way to get around the miles recorded, unless he grabbed a hold of someone's bumper and got towed, but that'd be pretty obvious, considering you'll more than likely be ahead of him first and he'd pass you while holding onto a car.

You know, he may know you're better than him and be saying all this stuff just for kicks, to see how far he can push you before you catch on.