A wider tire doesn't have more rolling resistance, and the weight is a miniscule effect on overall momentum. The big factor is the hysteresis of the tire you are running, and the heavy duty AT/MT tires tend to have a lot. There's a big variation though, and it's nearly impossible to find data. The Hankook ATMs I bought are fortunately good in this respect. MPGs actually improved compared to the stock Bridgestone street tires. Break-in might have helped some, plus I didn't lift mine at all.You are going to lose some fuel economy just turning the extra weight, now factor in the increased rolling resistance of the wider tire, you will notice the difference in economy.