Drag (friction, not sartorial eccentricity)
Further to the Loon's comments above, tyre pressues have a significant effect on how much fuel you burn. I've seen a noticeable difference between brands of tyre, some eco tyres save me 2-3mpg. That's far more energy than would be saved turning your headlights off.

Harder compound rubber in the tyres means they have less rolling resistence at the expense of outright grip. If you're not of the "drive it like you stole it" school of motoring, you won't notice any difference.

*edit* Can't see any fault with Enzyme's scientific theory!
(Bicycle Repairman "you're also a bit of a wanker", Mon 27 Oct 2008, 16:25, Reply)