My logic is you are basically riding on a shorter but wider tire when it is under inflated. Think of it on a bigger scale, say this truck for instance.. right now that tire is inflated so the rim is about to the top of the hood, now imagine it was only inflated so the rim cam up to the bottom of the bumper. You would not be going nearly as fast as the speedo indicates because that speedo is calculating speed for a 10 foot tire but it is only inflated to 8 feet now. Does this make sense? This would be much more minuscule on a car but it is still a difference in speed. This is also why when you are buying new wheels for your car, if you go up in size on the wheel you go down in size on the tire. But in this case, the wheel stays the same size but the tire is now smaller in diameter. Its all about the diameter, not the circumference.