Rear End Gear Identification

I have a 1985 Iroc Z. I wanted to put a set of new rear end gears in it to give it that extra umph at the start, it is basically all stock still, but how do I go about finding out what kind of gears are in it now, the tag that is usually on the dif cover is gone. so what should I do, also I wanted to know which gears would be better for me to use the 373's or the 410's?

Response from Art Housers Rear End Service:

You really should open the rear and count teeth to determine the gear and to see if a is in it. Unless you owned the vehicle from day one, the you have no guarantee that the original parts are still there.

I'm sure everyone knows that factory specs are not always the best specs for one's one use.

Many F-body owners ask about 12-bolt rears. They were not made for vehicles 1982 and up and will not work for them. This is why most upgrades result in a 9-inch Ford conversion rear. There is an occasional option, which is difficult to find and consequently more expensive.

Quick glance identification: * 8.2" or 8.5" rear? Check the head bolts. 8.2" uses 9/16" wrench and 8/5"
used 3/4" wrench.

I will send you more details once I catch up the latest bombardment of messages.

For Willard Lockerman and others with the odd rear...

Noticeably different with 9 bolts instead of 10
Length 7.75 inches
Made by Borg Warner
Limited slip has cone-clutch instead of disc-clutch
Roller bearings are tapered not straight

Produced for GM's Australian cars, especially Holden in late 80's. It appeared as option in some 1987 and other late Camaros (L98 and G92).

Ironically, the Australians have trouble finding it in their own country or find it quite expensive. Art has used and new parts available for them.

Other distinguishable marks that may have been changed since:
* painted black from factory (as opposed to unpainted)
* Borg Warner's logo cast at bottom of differential plate (Good luck seeing

Response from Art Housers Rear End Service:

Remind readers NOT to rely on VIN numbers or axle housing codes to determine what is in the rear.

As stated on our site, "Year stamp was rarely used, since the manufacturers used parts from one year for several years over time. As for center section, these parts were often changed, sometimes before initial auto purchase. Sheer waste of time. Just bite the bullet and open the rear."

Three factors to consider when choosing gear ratio:
1.. motor power
2.. tire size (if not stock)
3.. driving application (city, highway, racing)
Brand of gears is often a matter of preference. We like Dana/Auburn. Motive Gear is softer than others, so the grab well at first but tend to wear down faster. Motive will do for cruising use, but I wouldn't recommend it for racing.

We agree with advice as stated 3.73 is a good move.

Art Houser's Rear End Service
and High Performance Parts
Phone 1-888-560-2127

Another bit of help is out of my own shop manual, I found the actual ID codes and their physical location.


1. Raise back of car off ground with transmission in neutral so driveshaft is allowed to turn. Make sure
    the E-brake is disengaged.
2. While observing the driveshaft, grab hold of one of the back wheels and turn it. If the driveshaft spins
    not quite 3 revolutions per 1 revolution of the wheel, you've got a 2.73:1 ratio. If the drivshaft turns
    just over 3 revolutions, the ratio could be a 3.07:1-3.23:1. If the driveshaft turns almost 3 1/2
    revolutions, its probably a 3.42:1, and on like that.

The reason for the above is I've not yet been able to find the embossed axle code where they show in the shop manual. So the above is a quick and dirty way of getting a idea of whats inside the axle.

Steve from Roanoke, VA answers:

The 1st thing you should do is find out what gears you have, if your tag is gone you can always go by your vin number or the sticker in your glove box with the option codes listed on it. If you have by any chance (like my 85 iroc) the 3:42.1 gear ratio, then 3:73's are almost a waste of money. Because you won't hardly notice the difference. The only gear you can get is the 4:11 ratio to notice a good difference, although I'm told it will kill your gas mileage. I am still contemplating doing this myself. If, however you don't have the 3:42.1 gear ratio from stock, then 3:73's are a good way to go. If you have anything lower than 3:42's, then the 3:73's will make a good noticable difference, and they are a good highway gear.