Let’s face it: the rear differential in our classic cars is one of those items we seldom spend much time thinking about until something goes wrong. Except for the performance-minded enthusiasts who spend time calculating tire size, gear ratios, and engine RPM to find the best combination to suit their driving needs, the rear differential just isn’t often thought about.
One popular differential upgrade is to convert from an open differential to a limited-slip differential—or as we used to call it back in the day: switching from a one-wheel wonder to a Positraction. The term Positraction (or Posi or even Positive-Traction) is a GM brand name for its limited-slip differentials, and even though they stopped using the term in the 1980s, over time the name has become synonymous with this style of rear differential.
Although Ford and Mopar had their versions of limited-slip differentials and other manufacturers as well with names like Sure-Grip, Twin Traction, Twin Grip, Powr-Lok, Saf-T-Track, Trac-Loc, and even just Limited Slip, they all offered better traction and performance.
Despite the better traction and performance benefits limited-slip differentials offer, there is also a certain amount of expectation from the performance crowd to perform two-wheel smokey burnouts as opposed to receiving the ridicule of showing up with a one-wheel wonder (funny how performance cars make grown men 16 years old again).
Upgrading from an open differential to a limited-slip differential will make a noticeable difference in how your car performs, but it’s also a good time to reevaluate the gearing of your rear axle and make changes based on your driving needs.
Low rear-axle gears will help launch your car off the line quicker and get your car into its sweet spot in the RPM range a lot faster, while higher gears will sacrifice some off-the-line performance but reward you with lower engine RPMs on the highway and better fuel economy. Finding the sweet spot in gearing depends largely on each individual’s needs, but if you’re thinking of making a change, it’s a good idea to do it in conjunction with a limited-slip upgrade.
We join Mark and Gary Simpson in the shop as they take on upgrading the rear differential in an 8-inch Ford rear axle under a 1967 Ford Mustang. They’ll be installing a new third-member from Quick Performance with TrueTrac limited slip and upgrading the gearing from 3.00:1 to 3.40:1, giving this ride a little snappier feel without sacrificing too much fuel economy.