Rolling Resistance and tread depth....

Well, have you ever taken a set of drives from new to worn out and noticed how fuel economy sharply increased toward the very end of a tire's life? Part of the reason is that the tire gets smaller and registers more miles than actually driven but that's only about 2%, the rest is a decrease in rolling resistance resulting in increased MPG.

But unless you're the kind of person that measures every tankful and keeps a running total, you may not notice any change at all.