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I'm looking to buy a truck with cummins engine, automatic transmission.
I haul 40-45 klbs 80% of times and 85+ of times I run on flat roads of Midwest. What I need is the ratio of rear deferential to get the best fuel mileage.
With a 10 speed id go with 3.55. With a 13 or 18 u can go 3.36 and drop out of OD to keep rpm up. Tire height height makes quite a difference too. For example squishes 9900i has 11r24 tires with 3.55s. My truck has 3.36s and 11r22 same trannys. We pull about the same rpm at the same speed.
If you going to be heavy a lot, gearing it as low as you can get away with vs. the speeds you will be running will work best. (Ie. 3.70 will be better than 3.55's). Here is a chart and some discussion for reference.

i have a 13 speed with 3.36 rears and while it gets the job done it's NOT the best for fuel efficient cruising at good rpm's. At 62 mph I'm doing 1550 rpm in 12th gear or 1850 rpm in 11th gear. Gotta go over 72 mph to even THINK about 13th gear over 1500 rpm. If I ever have the opportunity I'd love to switch to either a 3.55 or 3.70.

If you're gonna be heavy like that then a 13 or even an 18 speed would be the best choice. They give you the extra gears when you need them and you can always skip around when you don't.
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