Weight is far worse than size. My 35's lost me zero MPGs and drive like stock because they're so light...of course that's with a calibrated speedo (if you don't calibrate you will see a lower than actual indicated MPG). As soon as I put on a 200lb bumper/winch combo I lost about 1MPG. When I had my 2010 Tahoe, I swapped out the stock 31" Duelers for 33" E-load Duratracs (because E-load was all they made back then) and lost 2.5mpg. Later when the D-Load Duratracs were released, I swapped out to those - same size - and got 1.5mpg back. Weight is a big deal, especially in tires. Also, you may not DIRECTLY care about fuel mileage, but you do care about it because it's an indicator of how your truck drives - when you lose MPGs because you added weight, you feel it in the performance of the truck too. It's not directly because of the lost MPGs, it's because of the weight that lost you those MPGs.