Something that we don't always know is when our tires are under-inflated. Some people check their tires regularly. If you're one of them, you can stop reading now. For the rest of you, if you'll take a moment to familiarize yourself with the orangish shape above, you soon will be able to tell when your tires are plumping out, as all new cars, starting with 2008 models, will have TPMS, a tire pressure monitoring system. When you need air in the tires, the symbol above will appear in the dashboard, telling you it's time to visit a pump (BTW, is there a site that tells you which gas stations still offer Free Air pumps? Because there totally should be).
The Auto Alliance is promoting TPMS as a new safety feature, but it's obviously more than that. Following a long spiel about how flat-ish tires can be dangerous to drive on, the Alliance's press release announcing the standard TPMS systems says that fuel efficiency is reduced one percent for every three-PSI under-inflation. Don't make me repeat that.
[Source: Alliance of Automobile Manufacturers]