Not toooooo sure that is the way it works. If you go much larger than stock you will prolly lose some mpg, though may not be to noticable (more mass = more hp to get to speed and sustain). I have 305's on 16's on mine stock and work fine, though that size limits your manufacture choices. I am getting 16ish in town and 20ish highway, not drivin like an a$%.
In theory, the taller the tire the less times it has to rotate per mile, allowing you to go further on the same amount of fuel The flip side is, if you go too big, then you lose power/mileage compensating for the extra rotational mass. If you go bigger, remember to re-calibrate the speedo.
nyteshades nailed it. Calibrate the speedo, and remember that taller tires on the same rim= more rubber in the tire=more rotational mass=less economy. Some sites list tire weight in their specs, so you can try to stay close to your original weight when shopping, or go with an aftermarket wheel/tire combo that makes up for the tire weight gain with wheel weight loss. Of course, at that point, the cost of the package offsets whatever fuel economy savings you may see for a couple of years!
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