mile driven. Part of what makes bigger tires seem like they get worse mpg's is that the odometer doesn't show as many miles driven. If your tires are 5% bigger but result in 5% worse gas mileage it's actually a wash. It didn't cost you any more to get from point A to point B, even though it might feel like it did. Alternatively, with bigger tires you can drive further (more actual miles) before your warranty expires and your vehicle will have fewer miles showing on the odometer when you go to sell it.