Here's a quick rearend primer...from memory, incomplete, but hopefully
I never mess around with trying to find the codes...it's much easier to
just turn the pinion and count the turns to get one revolution of the
wheels (or two revolutions if only one wheel is turning). The rearend
ratio is simply the number of turns of the pinion to one turn of both
There are lots of rearends available! Some fit better than others. For
6 lug rearends, the 64-69 1/2 ton 2wd Chevy (and some GMC) trucks have a
12 bolt rear, about 60-61" wide (I measure them where the wheels bolt
on). The 70 2wd and 70-81 4wd have a 6 lug 12 bolt that is a couple
inches wider, but will fit under a Task Force truck as long as you don't
get too wild with wheel/tire width. The 12 bolt truck rearends from the
60s usually had 3.70 gears, which is a bit much for extended highway
driving at 75 mph. You can swap the differential carrier and
ring/pinion gears from a later TRUCK or VAN 12 bolt, the 70s trucks
usually had 3.08 or 3.40 gears. Of course, you'll need to know how to
set up ring/pinion gears, bearing preload, and all that other neat stuff
to do this. These 60s truck 12 bolt rearends are from coil spring
trucks, so the perches are in the wrong place, and they have some extra
stuff on the housing for a panhard bar, which you won't need with leaf
Most 60s GMC trucks, and some Chevys, had Dana 44 or Dana 60 rearends,
and leaf springs. These rearends are 6 lug, about 60" wide, and will
bolt into a Task Force truck (with some fudging, the perches are about
1/2" too far apart). The brakes use different parking brake cables,
which is a problem, and the drums have a different offset. Gear ratios
range from 3.21 to 3.92 normally, the 3.21 is found behind some GMC V-6
engines, and is a nice ratio for an oletruck. The Dana 44 uses a
standard Chevy 1310 yoke, but the Dana 60 uses a heavy duty yoke, which
takes a larger 1350 series ujoint. A similar Dana 40 rearend was
availalbe as an option in 59 Chevy trucks, equipped with
positraction...rare, and with a 3.92 ratio. I have one of these
rearends, it was in my extended cab truck...I'm saving it for when I
build a race truck (one of these years).
The 71 and newer 2wd trucks use a 5 lug 5" bolt pattern, same as the
71-76 full size cars. There is not much available in the way of
matching front brake rotors that will fit an old truck, so this rearend
is not too popular a swap, unless you get the 70-up 4wd 6 lug axles.
Chevy also used 10 bolt rearends in lots of cars, in varying widths. A
common one is the 61" wide 70s vintage Camaro/Chevelle/Nova (and clones)
8.5" rearend. In Camaros/Novas it has leaf springs, but the perches
won't work with old truck springs. In Chevelles, it has coil springs,
with lots of extra brackets to cut off, and the cast in mounting "eyes"
on top, where the upper control arm bushings fit. Wheel bolt pattern is
4.75", and there are front disk brake kits available to match, or you
can use the 53-54 car front hubs/drums on the original 50s truck brakes,
to get the same bolt pattern front/rear. This is a good rearend for
using in an old truck, the ratios are commonly 2.73-3.36, with higher
gears available. The axle shafts are usually worn out in these things,
so check them (pull the cover, lockpin, and C clips out to get them
loose) where they ride in the wheel bearings...before buying the
The 55-57 Chevy car rearend is almost a bolt in, the perches need to be
relocated to the top of the housing though. It has the 4.75" bolt
pattern, and ratios are usually 3.55-3.73, but sometimes you'll find a
The 78-88 mid size GM cars use a 7.5" 10 bolt, which is about 58/5"
wide. This rearend would work well in an early (older than 47) truck,
but it is quite light duty. Ratios are usually 2.29-3.23, which means
you want some torque in the engine to pull it...but rpm will be low.
This is a coil spring rearend, with all the mounting brackets that need
to be cut off.
The Ford 8" and 9" rearends come in a whole bunch of varieties, and only
a few are usefull in oletrucks. The late 50s full size cars have a nice
one, but it's not easy to find brake hardware, drum, etc anymore. The
mid 60s midsize cars, such as the Fairlane, have a rearend that is about
the right width, and would be a good candidate...if you can find one.
The Maverick/Granada is very narrow, about 56.5", and would not fit
anything but a very early truck, or perhaps a tubbed Pro Street truck.
The 60s full size Ford car rearends are usually too wide, but if you
find one, measure it and see! Most earlier full size, and all mid size,
cars use the 4.5" bolt pattern, while later full size cars use the 5"
pattern. The Ford trucks have a big 5 on 5.5" pattern, with a big
center that takes some work to redrill to anything Chevy. Ratios range
from 2.47 to 4.30, and the axles come in 28 or 31 spline varieties.
There is lots of interchange possible with the 9" rearend, which is why
it's so popular...I have one in my 55 Belair, and in my 57 Suburban
chassis. Unfortunately, they are getting expensive, especially when you
have to shop around for just the right one that will fit.
So...if you don't know if a rearend will fit, or what it came from, get
out your tape measure and see for yourself! The most important things
are overall width, and wheel bolt pattern...then concern yourself with
ratio. I like a 2.75-3.0 ratio for a V-8 powered truck that will spend
lots of time on the highway; about 3.2-3.7 for a 6 cylinder or small V-8
for in town and limited highway use, and the stock 3.9-4.56 ratio is
great if you have to use your truck to haul lotsa weight, and you don't
have much engine...just don't plan on going much faster than 55!
Date: Mon, 1 Nov 1999 13:00:00 -0600
From: Terry Stellman <STELLMAN@noex.com>
Subject: RE: [oletrucks] Rear end Ratio
There should be a number stamped on the axle housing tube that you can
up to tell you what the ratio is and what it came out of.
Missouri City, Texas
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959