I had a bone stock 2006 2500 with 265/70/17 BFG All Terrains load range E tires and consistintly got 18-20mpg. I added the KORE HD leveling springs and Toyo ATs in the 285/70/17 load range D and are now getting 3-4 miles lower. I really like the way the truck looks, but dang with the price of fuel something has got to give. Is it the lift or the tires?

 

I would say both , the lift gives more air resistance , bigger tires , more effort to spin .
Set it up like a Nascar , low to the ground and lots of ground effects .
Looks are just that , learn to like the looks of anything that gives better performance , or like the looks and forget the millage .
I always trying to have my cake & eat it too .

 

Have you recalibrated your speedo to accomodate them spinning less times per mile? Worn out 265's to new 285's is a pretty good little jump. Your mileage driven may be a little short. Do you have them pressured up to the max? D's have softer sidewalls and will have more rolling resistance, especially if not fully inflated.

 

I had the original Ameritrac tires on my dually, pushing 100,000 miles on them and the wear bars were showing. I was getting killer mileage on the freeway set up at 55-60 on cruise control and when I put on a new set of tires (Pirelli Scorpion) the mileage dropped 3-5 mpg! It's all about the rolling friction. A traction tire gets less mileage than a basically 'slick' hard old tire.