Well from my experience the gas mileage is relatively the same. The tires don't make THAT big of a difference. It's really the odo reading that changes. I live in Las Vegas for a year and would go back home to So Cal every other weekend. This is when I had my completely stock Ironman, my odo would read 300 miles exactly from Point A to Point B. Making the same exact stops and the same exact drive in the Proton with 16" alloys and 32" BFG AT's my odo was off by about 40 miles. But yet I made it home on the same amount of gas I have always used...

Just food for thought...