Michael Johnson Associate Editor
November 20, 2006
Not that you need a steeper set of gears to do a burnout, but it certainly doesn't hurt. We're just glad car owner Mike Zamboni knew how to do a burnout. At the time of the gear swap, Mike's '05 Mustang GT had Falken wheels and tires, a C&L Performance cold-air kit, a DiabloSport custom tune, and a plethora of BMR Fabrication suspension pieces.

Horse Sense: Michael Zamboni, the owner of the '05 Mustang GT featured here, didn't wait long to dig further into the car. He added a ProCharger P-1SC in an initial attempt to go fast, but too many dragstrip passes spelled the end of the original Three-Valve. Michael now has a new engine ready for more boost, and that's good since he's stepping up to a ProCharger F-1 supercharger for a shot at the nines.

With the introduction of a new Mustang, there's something intimidating about getting into the car and making it faster. When the fuel-injected '86 GT was introduced, supposedly the age of hot-rodding was over. Obviously, that was incorrect, as EFI opened up a whole new performance world that still thrives today. Another intimidating factor was thrown into the mix when mass air meters became standard on all Mustangs in 1989. Then came electronic speedometers, which threw everyone for a loop, especially when it came to gears-every Mustang enthusiast's favorite swap. Each new Mustang always seems more difficult to modify at first, but when it's all said and done, it's still comprised of nuts and bolts, and it still has gears. Sometimes, the aftermarket has to spend time working itself into the computers to enable us to play nice with the electronics.

We've done a million gear swaps in these pages, but to the rookies among us who have no clue why anyone would want to change the gears in a Mustang, here's why. A lower (numerically higher) gearset gets the engine into its powerband quicker than a higher (numerically lower) set of gears. This is known as torque multiplication. With a lower gearset, the engine's horsepower and torque are multiplied at a more rapid pace when compared with a lower set of gears. A 3.73-4.30 gearset accelerates a car more rapidly than a 2.73-3.27 ratio. Consequently, a higher gear ratio allows your Mustang to run at a lower rpm on the highway. Finding the right balance between acceleration and highway cruising rpm is our quest with every gear swap.

With today's overdrive transmissions, however, it's fairly easy to step up to a steeper gear without worrying about over-revving the engine on the highway. Every Mustang enthusiast has a different view on which rear gear is perfect. Some want the everyday friendliness of a 3.55-3.73 gear, while some want to go all out with a 4.10-4.30 or lower gear. If your Mustang rarely sees highway use, you can get away with a low gear, but if you do both city and highway driving, a 3.73 gear would be perfect in our opinion.

Of course, what do we know? Mike Zamboni, the owner of the '05 Mustang GT on which we're performing this swap, initially went straight for the jugular with 4.30 gears. In our defense, Mike's main goal is to drag-race the car, with a bit of street action thrown in the mix. Mike has an enclosed trailer to haul the Mustang, so we don't think his thought process fits every Mustang enthusiast. Our best advice is to ride in a Mustang like yours that's had its gears swapped and see how you like it. One thing's for sure-you'll be hooked on torque multiplication before the end of First gear.

We're doing this buildup because Mike exploded the Mustang GT's rear with a 4.30 gear and another aftermarket differential prior to this round of modifications. When he exploded the previous differential, he wanted one that would hold up to his abuse. He chose Detroit Locker's Truetrac, and we enjoyed one of them on a daily basis in our former Fox coupe, so we agreed with his choice. We reported on this swap in the May '05 issue ("Swapping Ends," p. 151) using the same components, even though we chose 3.73 gears. Mike chose Moser 31-spline axles and a step back to 4.10 gears. He believed the 4.10s would complement his Mustang GT better than the 4.30s-hence the gear swap at this time. He must have been on to something because after the rearend buildup, he visited Bradenton Motorsports Park and ran consistent 12.50s on Mickey Thompson E/T Street Radials. Mike also blistered a 1.33 short time with the radials. We know what you're thinking, but we've seen the timeslip.

This swap took place at BMR Fabrication in Thonotosassa, Florida, which is just a short ride from our Tampa offices. When we arrived, the man turning the wrenches, Dave Piercey from Mustang Performance, already had the housing gutted. [Perhaps Johnson was late?-Ed.] Even though some prefer to lay out a mesh pattern on the ring gear to set up the rearend, Dave likes to use a barrel gauge, or an OTC pinion-depth tool. The tool component on the right in this image is the actual depth-setting tool, and the bearing is the new pinion bearing. The new pinion bearing race is already in place within the housing, which is done prior to measuring the new pinion's depth. Dave will start the process by installing the depth-setting tool in the housing.

With the depth-setting tool in place, the cylinder is installed in the same manner as the differential would be- torque specs and all. You can see the depth-setting tool just past the cylinder.