I couldn't notice any changes in MPG or performance, because I never even drove my truck on the stock tires. I had them changed out before I took delivery and drove off the lot in my new truck.
A larger, heavier tire absolutely HAS to have an effect on MPG and performance. It's simple physics and no way around it. That said, it isn't much of a change and the 3.92's give good mechanical advantage. I would venture to guess, the difference is probably less than 1 MPG.
And, FWIW... My speedo is only off by about 1.5%, and the computer reported MPG is usually within 1 MPG of hand calculated. Those things are assuming stock tire size, so obviously, it really couldn't have made much difference.
Hi DAA....I might be actually sending this message to you twice because I don't think the first one actually sent....anyways...
So my new Ram is showing up in January, pretty much same rig as yours. Longhorn, 5.7, 8 speed and 3.92 rear end. I was already planning to go up 1" in tire size to 275/65/R20, and actually throw on the same wranglers as you. Did you notice any substantial difference in fuel mileage or performance in going with the slightly larger size? Any insight you have would be much appreciated!