Quote:
Originally Posted by Chevyman350
I understand that the cummins want 1500 - 1800 rpm for peak mileage.
I like to cruse interstate at 75 mph (Midwest is mostly 65 speed limit).
I will be using 225 70r 19.5" tires.
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I'd call Randy's Ring and Pinion. You may need more than just gears to make this work.
Info I found from Yukon Gear says they make 3.54 to 7.17 gears for the Dana-70HD axle...
For the Dana 70 with a 5/8" Pinion offset (70, 70-U, 70 HD) you can get gears for the 4.10 carrier in 3.54 to 4.10 and 4.56 carrier in 4.56 to 7.17
More info states that the non 5/8" pinion offset housing can be found with (3.00 or 2.73 ). IDK how you would ID one.
I ran some quick what-if calcs through my RPM spreadsheet. You may actually be one of those unusual cases that would be well served by using a Gear Vendors overdrive unit.
225 70 19.5 = 32" diameter. 4L80E OD gear is 0.75:1 with your present 3.73:1 axle gears you will turn 2,210 RPMs at 75MPH. 1,724 RPMs when you engage the 0.78:1 GV overdrive.
225 70 19.5 = 32" diameter. 4L80E OD gear is 0.75:1 . Change to 3.54:1 axle gears and you will turn 2,097 RPM at 75MPH. 1,636 RPMs when you engage the 0.78:1 GV overdrive.
225 70 19.5 = 32" diameter. 4L80E OD gear is 0.75:1 . Change to 3.21:1 axle gears and you will turn 1,902 RPM at 75MPH. That's not quite in your sweet spot and you loose some of your ease-of-launch with a heavy trailer that the shorter 3.73 gears give you.
An Allison 1000 6 speed would get you where you want to be too... $$$$$