Keeping your car's tire pressure at the optimal level is a small and easy eco-tip that would help save a lot of precious oil if everybody did it. The principle is simple: an under-inflated tire adds rolling resistance, so the engine has to work harder to move the car. Best way to understand it is to imagine pedaling on a bicycle that has almost flat tires. So it's easy to see why we strongly recommend that you buy an inexpensive tire gauge and make sure to check once in a while if your tires are at the recommended pressure.Once you own a tire-gauge, why not offer friends and relatives to check their tire pressure too? Tell them it will save them a bit on gas and I'm sure they'll get interested. And for those wanting a bit more fuel economy, you can try going a bit over the recommended pressure to reduce rolling resistance some more, but if you do, make sure to check for uneven wear, particularly in the middle; that means too much pressure and could shorten the life of your tires. Whatever you do, just make certain not to exceed the maximum inflation pressure noted on the tire.