This is exactly correct. Mechanical speedos are designed to show an approximation of speed and will always err on the side of caution by reading high - it has nothing to do with massaging MPG figures. If it were possible to be going faster than the indicated speed then manufacturers would be liable to paying for every speeding ticket ever issued. If I'm going through a fixed speed camera bang on the speed limit, I don't want to get nicked because my speedo under-read by a few mph.
A few years ago I took part in a charity 'run-what-you-brung' motorcycle event, which consisted of 2 miles of pristine runway, with a calibrated radar trap in the middle. Even though my fastest indicated speed was 180mph, the maximum speed recorded by the radar was only 165mph; a full 15mph out. That was on a Suzuki GSX-R750, but some other bikes had much bigger discrepancies, from speaking to the riders up to 20-25mph in some cases. Note that sports-bike riders generally don't care about mpg either.