Found this online today under "Ten Things You Should Never Do To Your Car"
Never Overfill the Tires to 'Get Better Gas Mileage'
The Internet is brimming with testimonials from people who claim they upped their mileage simply by inflating their tires to the maximum pressure listed on the sidewall. What they don't tell you about is the rougher ride, premature tire wear, longer stopping distances and increased repair costs due to worn-out suspension components. The recommended tire pressure for your car is listed on a placard inside the driver's door frame and it's based on vehicle weight along with the best possible handling. Inflating your tires to the maximum pressure listed on the tire is okay if you're hauling a very heavy load. But you must reduce the tire pressure to the recommended pressure once you remove the load. Driving a normal load on over-inflated tires reduces rolling resistance and that can increase your mileage slightly. That means you have less rubber in contact with the road, which reduces traction.
Over-inflation increases stopping distances, causes the tires to slip and hydroplane on wet roads and reduces the life of the tire. The much harder tires also absorb less impact vibration so they transmit more road shock to your entire suspension system, causing a rougher ride. Worse yet, the additional tire bounce wears out your car's struts, strut mounts, shocks, springs, ball joints and control arms much faster. Any mileage gains you get from decreased rolling resistance are more than offset by decreased safety factors and increased suspension repair costs.