I was thinking of asking this same question. It appears that the mileage estimates are all over the place. The realization that people have different driving habits accounts for alot of the differences, but that shouldn't make a difference of up to 7 mpg for highway driving. I recently had a new ECM installed in my car and my stop and go mileage went from 17-18 to 14-15. I haven't had a chance to check the highway mpg, but I expect it has also dropped. Is a there a "computer relearn", or some other factor(s) associated with a new ECM that affects gas mileage?