Quote:
Originally Posted by capnbass91
Smaller tires helped my mpg as well. I recently went from 17" to stock 15" and my avg mpg went from 18.8 to 19.3 (I do mostly city driving).
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If your tires are sized properly, you'll have nearly the same diameter tire on the different wheel sizes. This means the tires are not smaller on 15" wheels, they're actually "larger", as in there's more rubber for the same diameter. If they are not sized properly, and your tires actually are smaller in diameter, then your odometer will be wrong, displaying more miles than you're actually going. This would deliver an illusion of increased mpg.
Edit: If your new size is the stock size, I suspect the old size was too big, causing the odometer to display fewer miles than actual, delivering an illusion of decreased mpg. Now you're just back to normal, reading the correct mileage.