I'm currently running 5.13's with 38 ridgegrapplers and am sitting at around 15-17 mpg at 65 mph (little to no head wind).
Depending on your driving I found that moving to 5.13's in 8th gear has a final drive ratio similar to running with 4.10's in 7th (see photos). And with a good head wind or hill I still drop down to 7th even now. Due to the gladiator being so heavy and if your doing any towing, live in a hilly/mountainous area, and or deal with heavy head winds I recommend going with 5.13's.
Most recent example was on the way to the new years run due to high winds/hills is was stuck in 6-7 gear most of the trip trying to stay at 70 mph and that's with 5.13's not sure how I would feel at 4.88s.
I should also note that putting the rpms to low will put the 3.6 out of its powerband and usable torque and might even cause it to start hunting for gears more often.
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