Gearing
Home | Free Mods | Intake | Ported Heads & Manifold | Exhaust | Gears | Transmission | Brakes | Parasitic HP | Fuelling | Interior | Weight | Performance | HP Gains | PartsThe rear axle ring and pinion in a stock 1995 Mustang GT automatic has a ratio of 2.73:1 making the overall gearing ridiculously tall. The reason was to improve fuel economy during highway cruising (at 75mph in overdrive the engine loafs along at just 1800rpm) but the intergalactic gearing is detrimental to performance.
One of the most popular (and best bang per buck) performance modifications on a Mustang GT is to replace the stock ring and pinion with a numerically higher ratio version to shorten the overall gearing. Torque multiplication to the wheels is increased and at any given speed, the engine can operate at a more efficient rpm where it's producing more torque. For a Mustang GT automatic with 245/45R17 tires (25.7" diameter), the optimum ring and pinion ratio is 3.73:1.
Ring and Pinion
I chose the Ford Racing 3.73:1 ring and pinion (part no. M4209F373N) since this is reputedly the best on the market, and I had it installed professionally. This shortened the gearing by 27% and at 75mph in overdrive, the engine now spins at 2450rpm. Torque multiplication to the wheels has increased by 36% (the ring gear still has 41 teeth but the pinion now has 11 teeth instead of 15) and acceleration in each gear has dramatically improved. Acceleration in overdrive is now much stronger and the transmission no longer needs to downshift to scale a long highway incline.
This is a modification that I highly recommend even in a bone stock Mustang GT, especially when it's equipped with an AODE automatic transmission and a factory 2.73:1 ring and pinion. I love it!
Master Bearing Kit
The Yukon master bearing kit from Late Model Restoration Supply (part no. YK-F88) includes pinion and carrier bearings, pinion seal, rear end cover gasket, ring gear bolts, pinion nut, crush sleeve, pinion and carrier shims, loctite, marking compound, and instructions. These are necessary to ensure proper installation of the new ring and pinion and prevent premature wear.
Speedometer Correction Gear
In order to keep your speedometer functioning correctly, the proper speedometer gear should be installed after changing the ring and pinion ratio. According to this chart, the recommended speedometer gear for my 1995 Mustang GT automatic (8 tooth drive gear) with a 3.73:1 ring and pinion and 245/45R17 tires (785 revolutions per mile) is the Steeda 23 tooth black recalibration gear with part no. 392-STE-17271-J. After this gear was installed, my speedo was reading 6% too fast. Acceptable and easily correctable with larger diameter tires. For comparison, the stock speedometer gear has 18 teeth.
The Steeda 23 tooth white recalibration gear on the right (part no. 392-STE-17271-D) is for T5 5-speed manual Mustang GT's with a 3.73:1 ring and pinion.