Miles Cook
October 20, 2011

If the vintage Mustang you’re driving still has its original transmission, then your freeway performance is not anything like an ’83 or newer Fox, SN-95, or S197—all of which have an overdrive gear ratio for Fourth, Fifth, or Sixth gear, depending on if the car has an automatic or manual.

Whether manual or automatic, pretty much all vintage transmissions have a final drive ratio of 1:1. Even with a relatively tall 3.00:1 rear-axle gear ratio, you’re still spinning about 3,000 rpm—or maybe even more—just to maintain a 65-75-mph speed limit. If you’re trying to keep up with your buddies who are equipped with overdrive, you can forget about it. They’ll just pull away from you like you’re hauling lumber. If your vintage Mustang is an around-town grocery getter that makes it to an occasional cruise night on the weekends, then a final drive ratio of 1:1 probably doesn’t bother you very much.

On the other hand, if you like to drive your ’65-’73 Mustang on the open road, then the addition of overdrive will convert your car from a freeway buzz-bomb into a quiet and smooth ride than can accomplish a 500-mile day with ease. If you’ve never experienced a vintage Mustang or other early Ford with an overdrive gear, you should definitely try out a friend’s conversion because it will seem almost miraculous as the transmission shifts into top gear and the tachometer drops from 3,000 rpm or more down to around 2,000 rpm or less.

If you’ve had enough of the right lane and you’re ready to turn your car into a freeway flyer, then an upgrade to a manual-shift overdrive gearbox is the solution. There are several different types of five- and even six-speed manual transmissions out there, but the T-5, as used in ’83-’93 Fox-body Mustangs, is the most popular. For price and performance, it’s hard to beat this time-tested and continually improved gearbox.

One of the more interesting cars we’ve seen lately is our nearly perfect ’65 Mustang fastback subject car dressed up as a Shelby GT350 replica. The car was equipped with a C4 three-speed automatic, so it was an ideal candidate for a T-5 five-speed conversion. We were reluctant to take this car very far from home because of the 1:1 final drive ratio. An hour or so on California freeways, with the tach reading 3,300 rpm trying to stay in front of the trucks that were blowing by us, got old pretty fast.

We solved that problem by contacting Modern Driveline. The company is a recognized leader when it comes to vintage Mustang manual-transmission conversions, and owner Bruce Couture has carefully engineered the company’s T-5 conversion kits to install easily and fit right the first time. Only the most reliable components are used in the creation of its setups. Join us and we’ll show you how to install one in your ’65-’73 Mustang.

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