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I used a data sheet from cummins in order to compute my mileage so i know what kind of setup to use so it will be an optimum setup. According to the cummins data sheet, if I run 33", 3.07s, and an NV4500 so .76 OD, I will be getting around 17mpg. Does this sound resonable? I was expecting more, my big Dodge 2500 got over 20 with the 6BT... If so, should I add an Aux OD to drop the rpm's ?? BTW, I uploaded the data sheet I used if any of you would like to look...
 

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1989 Jeep Wagoneer, 360v8, 727, stock for now,
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I run 33's, NV4500 with 3.73's and am very pleased with the performance. It also is the same over all gearing used by Dodge, but on a smaller scale. I get 23 mpg in town and 28 or better on the hiway.
With the gearing you suggest you would be shifting down at every little hill or head wind.
 

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Best MPG is usually cruise speed (peak torque RPM). I'll just use 1800rpm as a baseline. Depending on wind resistance you'll probably get the best mileage at say 55 or 60mph. If that's your goal, you need to gear towards that speed range. Here's a calculator I found easy to use.

http://www.csgnetwork.com/multirpmcalc.html

I'm running 4.09's with between 33 and 35" tires. Overdrive is 30% in a 700R4
My crusing RPM's should be 1500-1800RPM between 55 and 65MPH.

With your setup I show you turning 1313 RPM @ 60mph (in overdrive) which would be a bit undergeared. 1875RPM with a 1:1 ratio (good)


Looking at that PDF file It appears the best torque (and least fuel consumption) is between 1500 and 1700RPM. Not sure where you figured 17mpg. Remember weight & wind resistance play a factor as well. The bigger factor though, is engine RPM's.
 
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