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Old 01-01-2005, 04:08 AM   #1
ElGracho
Gentleman Jim Driver
 
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Join Date: Nov 2002
Location: Poulsbo, WA
Posts: 1,553
Axle Ratio Madness!!!

This topic comes up pretty regularly here so I thought I'd give my 2 cents. Maybe this could be refined with everyone else's input and go in the FAQ page.

Many people get a new truck and want to know what they've ended up with. The engine and transmission are usually pretty obvious (though I know people who've sworn they had a 350 and ended up with a 305...) but the rear differential takes more investigation.

The first place most people go is the options sticker. This is a good place to start, but it isn't all-telling. First off, these trucks range from 14-32 years old. It's uncommon for one to have under 100k miles. In that time, axles wear out and get replaced and often with different axle ratios. So it's not uncommon for many of the parts in these rigs NOT to be the ones they came with from GM. If your truck does have the original axle, you can read the code on your options sticker, decode it using an RPO decoder such as this website and be done. This isn't always perfect either. Sometimes you don't find a rear axle ratio code, or sometimes it gives the code: GQ1. Those don't tell you much. GQ1 means 'standard axle ratio'. This is the axle ratio that was normally installed on your type of truck, with your suspension, your engine, and your transmission.

Below is a scan of one page of the 'power teams' listing from a 1979 Chevrolet Light Truck dealer sales album that gives examples of these codes. There are 5 pages of listings for this year and differences for all 17 years of production of these trucks. If you don't have a code, or it is GQ1, this would be the way to find out what it was originally. Note that GQ1 shows up under several different ratios, so it doesn't tell you anything specific without looking up what was standard for that 'power team'.



You can also determine the ratio installed in an axle housing by using the code stamped on the axle tubes. This assumes the gears haven't been changed in that particular housing of course... The code is stamped on the front of the passenger side axle tube about 4-6" from the center casting. I don't have the breakdown of the codes, but I'm sure someone on the forums does.

The best way to figure out your rear axle ratio is to get the necessary gear oil, a new gasket and some RTV and pull the rear cover. You can read the numbers GM stamped on the ring gear, divide the number of pinion gear teeth into the number of ring gear teeth and you have your ratio. This also give you a chance to check the health of your differential, look at the ring gear teeth, spider gears, cross shaft etc for wear. Any extra metal bits in the bottom of the diff is a sign of impending doom for that axle.

Below you can see a picture of the differential out of my 87 Chevrolet R10. Sorry for the poor quality, but the yellow circle shows what we are interested in. The numbers that we care about are the 12 and the 41. These are the numbers of teeth on the pinion and ring gears. By diving 41 by 12 we get 3.42 so these gears are 3.42:1 ratio.



Another method of estimating your rear axle ratio involves jacking up the truck, rotating the drive shaft, and counting the number of revolutions a tire makes. The makes some assumptions about both tires turning at the same rate or dividing the count in two etc. that leads to errors. Also, you are usually told to rotate the wheels one turn and count how many driveshaft revolutions you get. In theory, this works, but in reality, can you really tell the difference of .15 revolutions of the driveshaft? That would be the difference between 3.08 and 3.23 gears. Another cause for error is the backlash in all of the splined connections, spider gears, side gears, ring and pinion, u joints etc that can all make this method less than ideal. It can be done, but I'd recommend just getting inside the diff and taking a look. If you haven't done it already, it's worth the peace of mind from knowing what you have and what kind of shape the internals of your differential are in.
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Joe
'75 GMC Gentleman Jim
'84 Chev C10 Short Wide - Super duper plain (manual steering, manual brakes, no dome light, no cig lighter)
'85 Chev C10 Short Wide - Super plain Vortec 4.8 4L60E trans
also: '81 K30, '83 C30 Crew Dually, '84 M1028 CUCV, '85 M1009 CUCV, another '85 C10 SWB, '89 R3500 Flatbed
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