Mr. Boxy
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A study by Carnagie Melon University calculated a 0.4% reduction in fuel economy per PSI below the recommended pressure. That might not sound like much, but according to a 2007 report by the Government Accountability Office, 26% of cars and 30% of light trucks on the road have tires that are under-inflated, wasting an estimated 1.2 billion gallons of gasoline each year.
Imagine rolling a water balloon and a rubber ball across the floor. If you push both across the floor with an equal amount of force, the ball will roll further. Why? Since the balloon isn't perfectly round, it's constantly changing shape, flexing so the underside is flat.
Now, imagine rolling the same two objects down a rocky hill. The water balloon will stay close to the road because it can conform to the surface, while the ball will bounce off the first thing it hits.
If you look at a car tire, it's a lot like the water balloon. The pressure from the weight of the car flattens it where it touches the ground, and the less air the tire has, the larger this flat spot will be. The recommended tire pressure for your vehicle is based on engineering studied designed to give you the best fuel economy while still providing enough traction to drive safely. In other words, tire pressure is a compromise between having a tire like the balloon and one like the ball.
You can see this effect on this test rig, which simulates changes in tire pressure:
While ideal for regular driving, it is sometimes helpful to veer away from recommended tire pressures under certain circumstances. Off-road drivers will lower their tire pressure to a few pounds per square inch so the tread will wrap around rough terrain for a better grip, while autocross racers fill their tires to their limit to reduce sidewall flex during sharp turns.
 Posted 5344 day ago
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