Hmmm, I was just thinking the same when you posted this. If I over-inflate my tires I may actually get better mpg than either the nav system or the odometer is telling me.

There's an initial benefit due to the reduced rolling resistance of the harder tire (which will show up in mpg calcs) but a secondary bump from the increased effective diameter. If each revolution of the wheel carries me just 1% further then my mpg will be understated by 1%. That's a good thing in terms of the environment, less good if I want to know accurately what I'm getting.

Colin