Just traded up from my stock LT 275/70R18 to LT 275/65R20. Love the wheels, but have noticed MPG dropped about 3 and take-off HP/shifting seems to be different. Ford dealer say I need a computer module upgrade for $100, tire shop says nothing required. Any pros know the real deal?

 

You went from wheels/tires that use 3:31 gearing to wheels/tires that need 3:55 gearing.....so yes your power/mileage is going to be affected. The dealer can go in and change the tire size in the computer to the largest 18" tire that is offered by Ford but you can't change to the 20" sizes in the computer.....it will throw a CEL......been there done that.

 

What he said

Yeah, you've basically re-geared your truck, taller tho, and probably more rotational mass, so more throttle needed, and less revs recorded on the ODO.

http://www.1010tires.com/Tools/Tire-...0R18/275-65R20

Your new tires are almost an inch taller than your old ones, so you're not recording as many revs per mile, which is contributing to your lower MPGs, and they're probably heavier, rotational mass changes affect accel and decel.

What you're describing is a perfectly normal and expected scenario, but I'll never ditch my 20s for 18s

 

I do like the 20" much better. So is the concensus that I'll be wasting my time going to the dealership for the computer upgrade? The 65R20's have a smaller sidewall, so the total height change was negligable. I guess I thought with the total tire circumference being so close, that there would not be much change in MPG and HP.

 

revs per mile changed from 608.25 to 591.88, or just about 3% difference.

the computer thinks you have the 608.25 revs per mile, so you traveled 'less' with the new tires.. so miles are down and gas is the same..

as others have said, there aren't any good computer adjustment options. with the stock dealer tuning, I don't know if the aftermarket tuners can get it closer..