Your tyres are the only contact that your car has with the ground, however the number of clearly under inflated tyres I see on a daily basis still astonishes me. Not only do under inflated tyres decrease MPG by causing more rolling resistance, they are down right dangerous.
Mythbusters Research has shown that under inflated tyres can reduce MPG by up to 4%.
There really is no excuse for this lack approach to maintenance. 99% of petrol stations have the facilities to check and correct tyre pressures (when they are not out of order) so why not check them each time you fill up. Failing that, a tyre pressure gauge is only a few quid on ebay, a worthwhile investment.
From a Hypermilers perspective, tyre pressures are a big deal. Many of us choose to run slightly over inflated pressures but all withing the tyre manufactures guidelines. This decreases rolling resistance and tyre wall flex as well as improves MPG, however this is NOT at the expense of safety or uneven tyre wear.
Check out our Hypermiling videos page for the Mythbusters tyre pressure findings.