March 1, 2009
Once you have the pinion depth set with the correct shim (arrow), press the new inner bearing onto the gear. Our used OEM 3.55 gearset from an '04 Mach 1 installed in the differential housing using a 0.030-inch pinion shim, which is the nominal factory shim.

Overdrive Ratios Explained
To determine the final drive ratio in a constant mesh transmission for any gear other than when the input shaft is directly locked to the output shaft (Fourth gear in a T-5), you must multiply the input ratio and output ratio of the selected pair of gears that power is being transferred through. In a T-5, when Fifth gear is engaged, power is directed from the input shaft to the countershaft and then back from the countershaft to the mainshaft. To calculate the final overdrive ratio, divide the number of teeth on the driven gear into the number of teeth on the drive gear for each pair of gears and multiply the two ratios. For example, in a T-5 with a 2.95:1 First gear like we're using, the input shaft (drive gear) has 24 teeth and the corresponding gear on the countershaft (driven gear) has 31 teeth for a 1.292 input ratio. With the Astro Fifth gearset, the large gear splined on the countershaft (drive gear) has 38 teeth and the smaller gear splined onto the mainshaft (driven gear) has 23 teeth, for a 0.605 output ratio. Multiply 1.292 by 0.605 to arrive at the final 0.782:1 Fifth gear ratio. Since the input ratio changes when the First gear ratio is changed, the final OD ratio changes as well. So if the Astro gears are used in a T-5 with a 3.35:1 First gear, the final overdrive ratio becomes 0.894:1 (34/23 23/38).

The backlash between the ring-and-pinion gears is measured with a dial indicator. The factory specification is 0.008 to 0.015 inch, with 0.012 to 0.015 being the ideal range. Adjustments are made by moving the differential carrier side to side with shims placed between the carrier bearings and the ends of the axletubes. We hit the backlash right on the money at 0.013 inch.

Transmission Tactics
THE WORLD Class T-5 transmissions used in '84-'95 Mustangs have earned a reputation for being marginal in high-horsepower applications, especially when combined with sticky tires. While it is true that the stock Third gear is an especially weak link in the T-5, especially when abused, these transmissions do have some things going for them compared to some of the popular alternatives, especially in a relatively low-power application like our CMC Mustang. For one thing, they are very light, weighing in around 75 pounds. They are also smooth shifting, which is a big plus in road racing, where quick, precise downshifts are as important as lightning fast upshifts, if not more so. The CMC rules do not allow radical internal modifications to the transmission like Pro-Shifting or face-plating, but there are improvements that can be made to increase its usefulness and durability for road racing. Most important is swapping the gas-miser 0.62:1 overdrive Fifth gear ratio for something more useful, because at tracks with long straights, our relatively low-revving 5.0L really needs the extra legs. There are various options for upgrading the Fifth gear, but one that's simple and economical is the A-5 0.79 Fifth Gear Kit from Astro Performance, which installs without any modifications to the transmission case or main gearset. We also used a B&M Pro Ripper shifter and a stock clutch disc and pressure plate along with a Ford Racing aluminum driveshaft.

Building The Rear
OUR DONOR chassis came with a bare 8.8 rearend housing and a set of Moser 31-spline axles, which we put back together with a leftover set of factory 3.55 gears and a used Detroit True Trac differential we bought on eBay for \$263. Given our 5.0's 5,500-rpm redline, we chose 3.55s in lieu of the 3.73s we ran in our old car as a better all-around compromise for most of the tracks. With the 3.73s, we tended to run out of rpm too soon in too many places and were often caught between gears.

We had excellent results in our old car running a Detroit True Trac torque-sensing, limited-slip differential, so we bought a used one on eBay for the new car. Unlike a stock Traction-Lock, the True Trac is maintenance-free, with no friction plates to wear out. The True Trac uses parallel helical gears inside the case to lock the axles together when the differential senses one tire is slipping in relation to the other, transferring power to the tire with traction.

Building rearends or swapping gears is one of those jobs we've always left to a pro, but we figured it was finally time to learn how to do it. Since we already had most of the tools needed for the job-a dial indicator, torque wrenches, and a shop press-we decided to save some time and money. A few hours on the Internet unearthed several do-it-yourself write-ups and tech tips, and armed with that knowledge, plus a deluxe gear installation kit, we tackled the job over a weekend. As it turned out, the job was not nearly as difficult as expected.

There are two critical steps to setting up a ring-and-pinion properly: establishing pinion gear depth and setting ring gear backlash. Setting the pinion depth can be tricky without an expensive factory gauge, so you will need to do it by trial and error. To set up the gear and measure the resulting gear pattern and backlash in the housing, you may need to press the inner pinion bearing on and off the gear a couple of times. Rather than risk damaging the bearing, it's easier to buy a spare bearing and ream out the diameter of the inner race so it slip fits onto the pinion gear. This allows you to mock up the pinion shim as many times as needed.

The photo captions highlight the key steps, but we'd recommend having a factory service manual before you try it at home. However, if you have the right tools and follow the proper setup and adjustment procedures, this is not a very difficult job. After racing most of a season without any odd noises or other problems with the rearend, we figure we did it right!