...because of the mileage penalty incurred for the first 1000 or so miles until the new rubber gets broken in. After the break-in, the mileage will steadily increase over the course of the tire's life and will be at its highest when the wear marks appear on the treads. I've experienced this MPG drop every time I've had to replace the tires on my car. This time I replaced my Michelin RainForce (with 70,000 miles on them) with Michelin Harmony tires. They discontinued the RainForce and the Harmony is the most similar tire Michelin is currently selling. Its been getting harder to find tires in my car's size each time I've had to replace them. In the process or re-tiring my car, I went from around 52MPG for my past several tanks to 49MPG for my first tank on the new tires and this second tank looks like it will be about the same.

I suspect the mileage drop from new tires has several causes. The deeper treads on a new tire create more aerodynamic drag than the shallow dimples of a worn out tread (closer in form to the low drag dimples of a golf ball). Also as the thick rubber on a new tread flexes, it creates more heat then the thin rubber of a worn out tread. Rubber also gets harder as it ages and loses its plasticizers. A harder tire has less rolling resistance than a tire consisting of a softer rubber. But the drop in coasting ability was instantly apparent as soon as I started driving on the new tires.