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Old 05-17-2013, 12:00 PM   #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, 03:41 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, 04:26 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, 05:13 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, 07:35 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, 01:44 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, 04:34 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, 05:52 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, 12:42 PM   #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, 01:40 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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Old 08-01-2013, 10:07 AM   #11
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Just got a quote at $3K for gear change and $500 to pull, reinstall

Definitely not going to make this up on economy, but dropping the RPM's and postponing rebuild / preventing engine failure makes total $$ sense.

Plus our next trip is across Texas, New Mexico, Arizona and Nevada in August...definitely want to keep the heat load down and run the speed limit.
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Old 08-01-2013, 02:14 PM   #12
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What ratio are you going to put in?
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Old 08-01-2013, 02:50 PM   #13
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Looking at 3:55 we think it has 4:08 now...400 cummins with 9 spd eaton

thoughts?
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Old 08-01-2013, 03:17 PM   #14
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Lower numeric. I don't know anything though. I think you would want to figure out what gear would give you the best cruising rpm for your desired cruise speed? Like say you want to go 70 mph. Then work backwards (or actually forward) from there to figure it out. I've got 3.55 rear gears I think and at 70 mph I'm seeing 1450 rpms (about). I don't know what the 10th gear ratio is on a Eaton Autoshift 10speed is though. I wouldn't mind being closer to 1300 rpms at 70.
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Old 08-01-2013, 03:18 PM   #15
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Oh wait, I see from my signature I've got 3.42 rear gears. Go lower than that then.
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Old 08-01-2013, 03:34 PM   #16
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They have a gear and axle shop making the recommendation. I have asked for calcs and confirmation of my current ratio once they get the gears out...in 30yrs, you never know whats really been changed.

It does hurt to replace perfectly good parts that seem to be running perfectly....

I only pull 15K so I would rather err on the side of too high a ratio, may look at the next step down from 3:55
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Old 08-01-2013, 03:51 PM   #17
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Yes, that's why I'm thinking go lower numerically. With such light weight you're not going to have any trouble getting it moving. I'm at 29k lbs. fully loaded water and fuel. I never use 1st gear. More often I start out in 3rd gear. If I could start out in 4th I might do that. My son's truck is about 17k bobtail. He starts out in 5th or 6th when bobtailing. I think you could figure out the gears yourself since that shop doesn't seem to want to do it for you. Let's try to figure it out here. What size tires you got? Exactly what transmission is that? Is 9th gear 1 to 1? or some overdrive gear?
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Old 08-01-2013, 04:12 PM   #18
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That's the fly in the ointment...there are no build tags and no way to check ratios short of jacking up one wheel.
Since I have convinced myself that I need to change the ratios, I don't mind having to bench verify what is in there. I do appreciate your offer though.

My rig is just short of 21K with another 15k of trailer/landcruisers behind it. I never use 1st /2nd, even with the trailer.

If we knew trans ratios, alot of the questions would be answered. I ordered every manual I could from International. Seems 1984 literature was too old to get added to the net. They assumed not that many rigs were still on the road.
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Old 08-01-2013, 04:20 PM   #19
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Is it a Eaton Fuller trans? I found this info on them: Eaton Fuller 9-Speed Transmissions
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Old 08-01-2013, 05:11 PM   #20
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GREAT link!!

Says eaton on the top of the case...Seller told me Dayton...I will have to look for the model # and unfortunately the shop is 45 miles away....I can ck it in the am.
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