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Old 05-17-2013, 11:00 AM   #1
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Default Rear end gearing

Since my chasis was built in 1984, 55 MPH speed limit, I top out at 72. I'm considering a rear end gear change to help my top end and mpg.
400 cummins with a 9 speed eaton...pulls like a banshee and with my 15K of trailer and toys, I can motor past people uphill...till I run out of gear at 2100 rpm / 70-72 mph.
Thoughts? Am I on the right track? Anyone have any gear change experience?
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Old 05-17-2013, 02:41 PM   #2
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Yep, that's awful high rpm's I'd think. I'm turning 1500 rpm's at 72 mph. I don't know what the cost would be for a change though. You'd have to weigh that against the possible fuel savings to decide if it's worth it. I'd find a good independent truck repair place and see what they'd charge you for a gear change. I think mine is 3.55 ratio in case that helps you.
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Old 05-17-2013, 03:26 PM   #3
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Those were my thoughts...1984 it didnt need to run 75 down the freeway. i never use 1st-3rd gears so I could stand to lower the rear end ratio.

First step is to find out what ratio I currently have....since I can't jack up the back wheels to check myself, I will find a shop.
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Old 05-17-2013, 04:13 PM   #4
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I found my ratio listed on a options/build sticker behind my fuse box I think. Don't know if they did something like that in '84.
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Old 05-18-2013, 06:35 PM   #5
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Measure the diameter of the tires ... then do the math and calculate the ratio based on RPM and speed when you're in the 1:1 gear ... or any gear you know the ratio of.
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Old 05-20-2013, 12:44 PM   #6
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If you can get just one side off of the ground, you can still count the revolutions of the drive shaft to get the gear ratio. You just have to rotate that one tire 2 revolutions instead of one, and count the driveshaft revolutions.

Also, I have seen a stamped steel tag on some rearends, generally under one of the bolts holding the pig into the housing with the gear ratio stamped into it.
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Old 05-21-2013, 03:34 PM   #7
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That might work...I havent a clue on any of the gear ratios, so one wheel up two revs of the tire.
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Old 05-21-2013, 04:52 PM   #8
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I think you will find that it will take a LOT of miles (with the increased fuel economy) to recover the cost of a gear change. Unless you get lucky and find someone who wants to trade.
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Old 05-23-2013, 11:42 AM   #9
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Fuel economy is definitely a benefit, but the driving force is top speed and reducing the rpm load on the engine.
I tend to be a little heavy footed and literally pin it when the prevailing speed limit allows.
If I can drop the rpm and pick up the ability to run at the 75 MPH speed limit loaded without stressing the engine, I will be happy.
I currently get 6-8 mpg...and have zero aerodynamic advantage. However, I did remove the rooftop wing since down force is not an issue.
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Old 06-15-2013, 12:40 PM   #10
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I've got 3:23's in mine behind a 450 Mercedes and 12 speed Meritor
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