Have you recalibrated for the tire change (so mileage is accurate while attempting to figure correct mpg)? This does not sound right at all- I'm running 245's and am at around 2100 @ 70, not exactly sure at 75.

 

That rpm sounds about right to me, the smaller tire is rotating around a little faster than the bigger ones due to the smaller diameter.

 

Tire Size LT325/60R20 LR D
Revs/Mile at 45 mph 586
Overall Diameter 35.5
Section Width on Measuring Rim Width 13 on 9.5
weight rating 3195@50PSI
Tire Size LT265/70R17 LR E
Revs/Mile at 45MPH 654
Overall Diameter 31.8
Section Width on measuring Rim Width 10.7 on 8
weight rating 3195@80PSI
when comparing the tires i think you see why there is a difference in the way the truck opperates.

 

Mine runs about 1900 at 75ish , but I also have a six speed and 285s mileage is pretty decent between 18 and 20mpg. My 02 w/ 6.0 got about 12.5 with 285s at 75 pushin 2400rpm so I am happy with the Dmax, even though fuel is more:gearjamin

 

my truck with 33" tires and a 6SP Ally it runs 73-74MPH at 2,000 RPM's..

ON EDIT:with the 33" i run 80MPH at 2,000RPM's..i just checked that out last night because my earlier statement didn't seem quite right,guess thats what i get for going off my memory.

 

hmmm interesting...I run about 2000 rpms at about 72-73 with my 285s on and I only have a 5 speed. My speedo is recalibrated and verified with GPS. You want to see your mileage go up? Try slowing down. My last tank I didn't drive much faster than 60 mph (speed limit is 55) and I got 18.5 mpg. The previous tank I drove the same roads driving 65 I got 15.5 mpg. I had a lot of variables (changed programmer settings, used different fuel additive, and the possibility of not having blended fuel anymore) changed between last tank and this one, but I'm sure that slowing down helped a bunch.

 

This is what I was tought. The smaller the tire is better for mileage on city driving. The larger the tire, the better it is for highway driving. So you have to look at what you do more of. A big tire in the City will suck, and a smaller tire on long highway drives will suck.

 

I'd have to say I agree with that!! Smaller tire makes accelerating easier, using less fuel. Larger tires lower the RPM's at higher speed, using less fuel. Can't say I know from personal experience, but logically that makes perfect sense.

 

The 2200 rmps is spot on.. i just picked up an 05 lly and thought that running at 75 mph, rpms would be hella lower.. thought maybe the tranny wasnt grabbin 5th or something.. good to see im not alone.. that means nothings broke.. yet.