So I think I have the wheels and tires figured out.. It got me thinking though.. I average about 13.5-14.5 MPG with the stock 17" "offroad" tires...

I was wondering if I would see any noticeable drop going to 325/60/20s

 

I have 35x12.50-R17 mud terrains on my truck, and it gets about 14mpg right now. 20" rims will have more of an effect on rolling resistance compared to a 17" with the same diameter tire because the 20" rim place the heaviest part of the weight (the tire) farther from the axis. The farther weight is from the axis, the more force it takes to accelerate and slow that mass. You will loose MPG, but probably not too much, and after you re-gear for the tires you should get some of it back.

 

I'd guess about 2 to 3mpg less in stop and go city and 1 to 2mpg less on the hwy driving the speed limit. I agree with Tornadom it's the weight at all 4 corners that kills your mileage, and brake's!

 

So it will still be better than the cobra lol...

I was hoping to not have to re-gear will have to see.. I do have the optional lower ratio I want to say its 4.10 or 4.35 can't remember would have to look it up.

 

My hwy gas mileage is still 16-18mpg with the 35's. In city driving it drop's down to 12-13mpg. On the hwy I try not to exceed 70mph.

 

With my stockers (18") I'm around 14.5-15+



With my summers (33x12.50 20's) I'm around 12 city or hwy

 

Jeez mine is pretty much 13.6 city, highway, going up the mountain, flat land, towing... doesn't matter lol

 

My tires are not as wide as what you are thinking about, BFG 285/65/20 but mine dropped from 16 highway to about 14 at 75 mph. City driving was bad before and is about 11 all the time now.

 

I went with 325/60/20's and have not noticed my gas mileage change very much at all. I was around 13-14.5mpg. I am still in the same range.

 

If you have the lowest gear ratio that was offered on this body style F-150, it would be 4.10. Honestly, 4.10 wouldn't be terrible with the 35s, but you will notice a little off the line powe loss due to getting all of that weight moving. I haven't regeared my truck, and I have the crap-tastic 3.55 non-LS differentials. Heck, I've even towed a 6,000 travel trailed up hill and into the wind with my current setup (I locked it in 2nd and burned a gallon of gas evey 7 miles)

If it is open road and I can set the cruise control at 70-75 I have gotten 18mpg with the lift and tires.

 

I went from stock 20s average of 16 city and 18 hwy to 35s Now Im getting 14 city and 16 hwy. I have the 373 gears. You will notice a loss of fuel economy. I have not noticed a power loss but I lost about 70 miles t the tank. Good luck.

 

Im getting around 14.5 city babying bigtime and 13 or so driving normal. Highway around 16.5-17 at 70mph. Thats with 35x12.5x17 all terrains, duals, CAI, and an edge. Bottom line your going to lose something probably about 2 i'd guess.

 

I was consistent with 13.5-14mpg with mixed country roads/town driving on stock 20's w/3.73 gears, larger v8 gas engine (can't remember, duh). After 6" lift and 20" 35's i am approx down 2mpg (11.5-12)
EDIT: After I corrected my speedometer for larger tires, the mpg calculation was corrected. I'm netting out at about a loss of 1mpg. getting 12.5-13 with mixed driving.