For my Subaru it took more of an MPG hit with the roof basket than the tiresI honestly can't say, I'm sure the heavier tires do take a hit on mileage but with the sizing being off I really can't go by my previous estimations. According to the app Fuelly which I record all my fuel fillups, I filled up on September 16, 2 days after the install and at that time was averaging 8.1L/100 km. Today I topped up my gas and it said I beat my last mileage at 8.0L / 100kms. My overall best has been 7.3L / 100km. So I guess in US that would equate to approx. 30mpg overall, not sure if that is decent or not or if it gets better, I only have 3000kms.