8.8" Rear Axle and 3.73 gears

This is a used 8.8" rear axle/differential that was pulled off a 2000 Mustang GT.   It is considerably stronger than the 7.5" axle that comes stock on a V6 Mustang.   When I bought it, this axle had 3.55 aftermarket gears and a trak-lok that had been recently rebuilt with friction [F] and steel [S] plates in the order: F/S/F/S/F/S/F.   The stock order is F/S/S/F/S/S/F.   Increasing the number of friction plates improves the transmission of torque to both wheels.   The axle was pulled with its upper and lower control arms, but I discarded them.   The control arms on my V6 Mustang are identical and their bushings are in better shape.

  

The designation 8.8" refers to the diameter of the ring gear, but you can tell an 8.8" axle from an 7.5" axle without opening up the differential case.   Just look at the shape of the differential cover.   The 7.5" cover is rounded on the sides and flat at the top and bottom.   The 8.8" cover is more nearly square -- with flat spots on all four sides.  On a late model Mustang, the 8.8 axle will also have quad shock mounts.   They are not found on a 7.5 axle.

Ring Gear Teeth 30373639414141
Pinion Gear Teeth11121111111009
Axle Ratio 2.733.083.273.553.734.104.56

When you open up a differential, you can check the axle ratio by counting the teeth on the ring gear.   If they are Ford gears, you will probably also find the ratio stamped on the ring gear, as shown above.   In this case you can see that they are 3.55 gears.   This picture was taken before I had a differential shop install the new 3.73 gearset.

   

As part of the purchase of this axle I also got a new set of axle bearings and seals (left).   I borrowed a puller from Checker Auto to remove the old bearings and seals (center).   You need a bearing driver or a large socket to beat on when you install the new bearings.   I made a driver out of some 1-1/4" pipe fittings (right).   The pipe coupling has an O.D. of about 2-3/16", which is ideal for installing the 2-1/4" O.D. bearings.   I also found that heating up the end of the axle tube with a propane torch made the removal/replacement of bearings easier.

  

V6 Mustangs do not come with chassis brackets for quad shocks (aka rear axle dampers), but the threaded mounting holes are there.   I pulled a set of brackets (pictured above) off of a 1987 Mustang GT and mounted them on my car.   I did not pull the 16-yr old dampers.   Instead I bought some new Monro-Matic Plus Shocks (P/N 32355) (pictured above).

Shown above is the installed 8.8 axle and the passenger-side quad shock.   The brake calipers and rotors have not yet been put back on the car.

Note: If you've running aftermarket lower control arms, you probably don't need to install quad shocks.   Their sole purpose is to prevent axle-hop on hard launches, which can be a problem in cars with rubber-mounted stock suspension components.   Installation of aftermarket control arms with poly bushings or spherical bearings tends to eliminate that problem.


Removing/Installing the Rear Axle

Ford service bulleltin No. 00-23-1 describes the process of replacing a Mustang 7.5" axle with an 8.8" axle.   You can also find instructions (with good illustrations) in the Haynes and Chilton repair manuals.   The process is not difficult, but you'll welcome having a friend to help you wrestle the new axle into place and line up the control arm bolts.   (Matt, thanks a lot.)   Shown above is the 8.8" axle installed in my car.


Speedometer Error

When you change the rear axle ratio, you throw off the stock speedometer and odometer.   The formula to calculate this error is:   speedometer error = new axle ratio/stock axle ratio.   Using that formula, you can calculate that replacing stock 3.27 gears with 3.73 gears will cause the speedometer to read 1.14 times the old readings, or 14% high.

To correct your odo/speedo readings you will have to use one of the fixes listed in the chart below, which shows what will work with different model years and transmissions.

Odo/Speedo Correction94-98 All99-04 5-spd99-04 Auto
New speedo gearX  
SpeedCalXX 
Flash or Chip XX

On 94-98 cars you have a choice of either installing a recalibration gear in the speedometer drive or buying an electronic correction box like the $99 Dallas Mustang SpeedCal unit, shown below.   On 99-04 5-spds, you can use a SpeedCal or you can modify the car's computer with a chip or a reflash.   If you've got a 99-04 auto, your only choice is a chip or reflash.   I installed a SpeedCal on my '00 5-spd car.


The SpeedCal unit installs behind the shifter bezel and needs a +12V power supply.   The installation instructions say to hook it into the blue/white power line to the cigar lighter, which is hot at all times.   I chose to tap into the pink/black power line to pin 3 on the traction control switch.   It is only hot when the ignition is on (no battery drain at other times), and the SpeedCal works fine that way.

I found that with the SpeedCal I was able to correct my odometer to within 0.1% of true miles, as measured with my GPS receiver.   That's an error of only 1/100 mile (about 50 ft) every 10 miles.


Driving Impressions

11 Jan 04:   I'm still breaking in the new gears (short trips only, for the first 500 miles) but I'm very pleased with the way the car feels.   Much more pep during accelleration, and I find that I can now use 5th gear in around-town driving.   I'll be interested in seeing what effect the new gears have on E.T. at the strip, and on my highway miles per gallon.   But that will have to wait until I've finished the break in process.   I'll also say I'm glad I decided to have the gears professionally installed.   They are very quiet.

26 Mar 04:   The gears are well broken in and I finally made it to the track.   I improved on my previous best E.T. by 0.28 sec and my 60ft by 0.16 sec.   The gears do make a difference.   With 3.27's I was crossing the line still pulling in 3rd gear.   Now I'm well into 4th.   Previously, around town the car drove like a 4-speed with overdrive. I seldom used 5th gear.   Now the car drives like a 5-speed.   I use 5th all the time.   My gas mileage is about what it's always been.   I think that's because I'm using 5th more, and actually turning fewer revs around town.   Great mod, and lots of fun.

Update

02 Oct 08:   After putting about 50,000 miles on the new gears, I changed the lubricant (80w-90 gear oil & a tube of friction modifier).  That gave me an opportunity to check the wear pattern on the gears.  Both the pinion and the ring gear look like they are wearing well.   The magnetic fill plug didn't have any metal particles attached to it.  And the old oil looked fairly clean.   All good news.