Regardless of which transmission tune you have, at any speed above 70 mph the transmission will be in 5th gear.

For mine, 75mph is 2100-2200rpm and 80mph is about 2300rpm.
I'm running a 245-75-16 tire which is about as tall as you would ever want to go on a KJ.

I was running a 215-85-16 tire a few years ago back and I really liked them but they didn't do to well on wet pavement.

The various GDE transmission tunes tinker with shift points and torque converter lockup so they can change rpm just not at those speeds. For example the basic tune lowers the "final shift" point from the post F-37 ca. 63 mph to ca. 58 mph and lets you hold that down to near 50mph. Hence if you have that GDE tune and have made that final shift then for speeds in the range of near 50mph up to ca. 63mph your rpms will be lower than a CRD with the F-37 tune but once above ca. 63mph there is no difference. Hope this makes sense.

Also if you think it thru that GDE tune only helps mpg for more or less in town driving where you are not above the final shift point.

Only simple way to lower engine RPM's is to run a taller tire! Taller tires lowers final drive ratio of engine speed vs ground speed....
245/75/16 requires some small lift (new springs) to clear body parts when turning...

It will be around 2200 RPM or so which is perfectly fine. These diesels redline at 4000 rpm but its not because they will explode or anything, its just because diesel fuel doesnt burn fast enough to provide much power above that. Plenty of people have run at 80 mph in these, including me. I drive all over Wyoming, Utah and Nevada, where its 5 hrs between patches of civilization and the speed limit is 80 mph and its totally fine. It doesn't sound like its stressed out, it sounds just like any other vehicle would at that speed. In fact its probably better because at that speed you are right in the peak torque range so it cruises along effortlessly.

I also dont think a bigger tire will pay off in increased mpg's or reduced stress on the motor. All it will do will satisfy you efforts to reduce rpms which don't really matter. Regearing the diffs to a lower ratio is the only way you could do it not take a hit from tire weight and drag.

After re-reading it again, that definitely makes sense. Thank you. Just to clarify, F-37 refers to the GDE trans eco tune?






Thanks guys. What kind of MPG's you getting on 16's at 80mph with an eco/hot tune?
Or to Mass-hole, what are you returning with regular tires at 80mph?

I too used to get around 30 mpg with the stock setup. With lift and bigger tires it's down about 5 mpg, throw on the Moabs with the BFG ATs and it goes down another 3 mpg. I think the increase in height and weight/inertia/rolling friction quickly outweighs the RPM benefit.