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Old 02-02-2016, 08:44 PM   #1
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Default Axle Ratio

I Have 1990 GMC 7500 Topkick, Gas with five speed. The box has been shortened 10 feet. iT has a 2 speed rear axle. I never checked the axle ratio when purchased. It turns out I have a 7.17 to 9.19 ratio. Going to haul a 35 foot fifth wheel. With this ratio I am turning 5000 rpm at 50 mph. What would be my best axle ratio.And can I use a axle from any other mdt or just from a GM model.
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Old 02-03-2016, 09:43 AM   #2
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Google - Axle Ratio RPM Calculator

also - differential gear ratio calculator
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Old 02-03-2016, 02:25 PM   #3
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Things you need to know:

-- where your engine makes torque and hp, (what rpm)
-- ratio of 5th gear in your transmission, (is it direct drive or over drive?)
-- what type of driving you plan to do, (highway, hills, etc)
-- your goals, (do you plan to tow the 5th wheel all of the time?, are you going for maximum fuel economy or do you want power?)

You could obtain a split rear end with faster gears, set up for highway mileage at a few hundred rpm's over peak torque in 5th gear with the High rear end ratio, (numerically lower). Then you'd always have the lower, (numerically lower) rear end ratio for towing or hills.

The split rear end is a real nice option, however it is also possible, (easy) to swap in a single speed rear end. If your 5th gear is in fact an over drive, you could set up things for towing in 4th a little over peak torque, then use the 5th for mileage.
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Old 02-03-2016, 09:12 PM   #4
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Thanks for all the info
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Old 02-03-2016, 10:04 PM   #5
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366 or 427? Back around then we were running those gas motors in our 6500 delivery trucks. We ran 6.14 ratios with the 427 and 6.50 ratios with the 366. The 6.50 ran about 3500 @ 55mph which was fine for an around town delivery truck. I currently have a 2003 6500 Topkick with an 8.1 gas to tow my fifth wheel. I have 5.57 gear (with a 5 speed allison w/over drive) and 235/80/22.5 tires. With that setup it runs 3400 in 4th gear (1:1) and 2800 in 5th (overdrive) at 65mph. With my 30k rig, that works out about right with the 8.1.

You can ignore your low range on the rearend for calculation purposes, just worry about high range which should be 1:1 and only the ring and pinion ratio will come into play there. Fifth on an old manual transmission like that is unlikely to be an overdrive, more likely 1:1. You could find the mfg and model of the transmission to research that for sure.

From our delivery trucks back then, GM used a few different axles so just because it is GM doesn't necessarily mean it is the same. Again find the mfg and model of the rearend. You may be able to find just the center section at a truck junkyard to change the ratio. It may also be easier to have a transmission shop swap out your ring and pinion only.

You will likely want a big block chevy to run around 3200-3500 rpm at cruising speed to have decent power, any less and you will be bogging it down anyway. I can only run my 8.1 in overdrive on flat ground, any kind of small grade and it wants to shift down anyway.

Regardless of all that, you are going to have way worse gas mileage than you think with that rig. Mine gets about 4 mpg at 30k total weight. I can squeeze a little more if I run it at 55 and ease it up the hills. My first suggestion would be get that thing up on the highway and burn up a couple of tanks of gas and see how bad it is before you sink a ton of money in the truck and find out you can't afford to drive it. Take it from experience, when gas was higher not much over a year ago mine was costing me almost $1 per mile. $1000 to get home from the Keys last year. My goal right now is to move up to a diesel semi chassis to at least double my mileage and tow easier. While I love my 6500 for towing (except the mileage) I now wish I'd gone straight to a semi in the first place instead of having to build a truck twice.
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