Epp my 6.5L with 4.56:1 and 4,000 pounds daily in the bed net me 12 to 14. At 70mph.plus its like 11mpg. Manual that had a .62 OD (aftermarket ) was 14 to 15.7. My other nv4500 got me 12 to 14.....i guess I need new rear gear
If you're hauling that kinda weight daily, I wouldn't want any smaller than 4.10s
Would rather have bigger gears and lighter weight diff fluid vs smaller gears. The smaller gears will be under a lot more stress and heavier fluid can only make up for so much of that.
Chnage your diff fluid to 80/90 and see what that gets ya. On the side of caution, have an infared thermometer to point and shoot at diff cover before and after fluid change to make sure they aren't drastically different under the same driving conditions.
My loaded long haul I mentioned I was able to get to ~75mph w/o OD, then while coasting poped column shifter into D and set cruise landed me right at 70 where I wanted. Only reason I did was cause route was so flat, otherwise would have stayed in 3 and just gobbled up fuel to save tranny the wear. It still probably took 3 miles to climb that high w/o flooring it. I didn't even try to exceed that speed, but doubt I could have. Certainly had ZERO passing power at that speed with that load my stock GM8 turbo haha. I got lucky in the fact that I only had to hit the brakes 1 or 2 times cause of traffic so didn't have to re-climb to that speed.
Pretty sure that trip is what did my leaf springs in though. She's always had the forward pitch where front it a bit lower than rear. Now she sits level with no load.
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