Found another way to save on gas.

[MetroMPG]

hate to revive this old debate, but i believe i have found some solid support for the issue of PSI and tire diameter/circumference.

more air pressure DOES slightly change the diameter of a tire. this difference is the basis of operation of some tire pressure monitoring systems, which detect pressure discrepancies based on input from the ABS wheel sensors (looking for different rotational speeds of the tires).

Taken from http://www.aa1car.com/library/tire_monitors.htm:

The other method of monitoring tire pressure does not require any additional hardware inside or outside the wheel. It is the "indirect" method. This approach makes use of the existing wheel speed sensors in the antilock brake (ABS) system. By modifying the ABS operating software, the ABS system learns the average speed at which each wheel rotates when traveling straight at a constant velocity, and detects low tires by comparing relative wheel speeds. The auto makers like this approach because it is a simple, cost-effective way to monitor tire pressure. But it has some limitations.

One is that it can't detect a low tire if two tires on the same axle are low, if two tires on the same side are low, or if all four tires are low by an equal amount. But it can detect one or more low tire if the rest are at or near the recommended inflation pressure.

Here's how the indirect ABS-based TPMS works. If a tire loses air, the diameter of the tire shrinks slightly causing it to turn at a slightly faster rate than the others. But on most applications, a tire has to loose anywhere from 8 to 14 pounds of air pressure before there's enough of a difference in diameter to be detected by the wheel speed sensors. It depends on the type of tires, tire diameter and aspect ratio, and the sensitivity of the ABS system.

Low profile tires with short stiff sidewalls change diameter less than tires with taller aspect ratios and more compliant sidewalls. Consequently, a loss of 10 psi in a low profile tire may only change the diameter of the tire less than 1 mm (0.040 inches). Most ABS systems cannot detect changes smaller than 1 mm. For this reason, indirect ABS-based tire pressure monitoring systems are not as sensitive as direct TPMS systems that have a pressure sensor inside each wheel.

ABS-based systems must also relearn the rotational "signature" of each tire when tires are replaced, repaired or rotated. This requires the driver to push a reset button on the dash or to follow a menu on the driver information display. It's also essential that the tires all be properly inflated before the recalibration procedure takes place -- otherwise the module won't recalibrate correctly and may not detect a low tire.