Always check pressure when the tires are cold, since driving even a couple of miles to the gas station can provide a false reading.
- Under-inflation generally provides a smoother ride, but it also causes tires to wear out at the sides, and it wastes gas because tires need more power to push the vehicle.
- Higher pressure generally results in improved steering response and fuel economy, but a stiffer ride, and it wears out the tread in the center.
Even the experts get it wrong. Top automotive website Edmunds.com tested vehicles driven by 212 of their employees and found a bunch substiantially under-inflated and wasting gas.
Edmunds.com (full disclosure here -- I write for them occasionally) discovered they would save nearly 6,000 gallons of gas a year and about $20,000 of their boss' money, by keeping tires properly inflated.
So go check your tires. It could make your great drive more fuel efficient.