Ring and Pinion Gears: The Ultimate Debate

Without question, Ford Mustangs have a pep to their gallop, but what if you want your pony to stampede? A steeper ring and pinion gear set in a traction lock differential will make any Mustang accelerate at a quicker rate by mulitplying torque according to the gear ratio.

Many enthusiasts stay away from gear changes because they do not want to hinder driveability, gas milage, and they have been misinformed about how the gears really work.

For those who do decide to swap gears in their Mustangs, the debate often comes down to 3.73:1 vs. 4.10:1 gear ratios. Both of these ratios are steeper than stock, but 4.10:1 is a steeper gear than the subtle increase of 3.73:1 over the stock 3.27:1 ratio. Many enthusiasts purchasing a gear for a street car choose the 3.73:1 because they are afraid a 4.10:1 ratio is not “streetable.” This article will prove this misconception wrong.

For a Naturally Aspirated (N/A) car, 4.10:1 is the only way to go, regardless of amount of time the car sees the highway. The MPG difference between 4.10:1 and 3.73:1 is at normal mixed driving 1 MPG per tank. However, the power difference between 4.10:1 and 3.73:1 is substantial!

Gears function by multiplying engine torque to the rear wheels. For a basic example, suppose an engine makes 300 ft./lbs. of torque at peak rpm. This engine with 4.10:1 ring and pinion gears makes a theoretical torque of 1230. Now, the same engine with 3.73:1 ring and pinion gears makes a theoretical torque of 1119. This difference eventually reaches the ground through the wheels, and by sacrificing 1 MPG on a N/A street car, the rewards far exceede the risks!

More misconception about rear end gears comes from the old Pushrod 5.0L guys. Mechanically, Pushrod engines produce more torque at a lower rpm than do the higher revving Modular engines of today. Since the Modular engines get into the torque powerband at higher revs, it is more streetable to run a steeper gear than it is for the old Pushrod engines.

Now that we know how gears function, let’s discuss what gears to get and where to have them installed. Ford Motor Sport (FMS), Ford Racing Performance Parts (FRPP), and Motive Performance make quality ring and pinion gear sets for the Ford Mustangs. After installing a gear set, a recalibration of the speedometer is needed. On 1996-1998 manual transmission Mustangs, a Speedometer Gear or Speedometer Calibration (Speedcal) will adjust your speedometer to the gear you have installed. For 1999+ manual transmission Mustangs, a Speedcal is the only option. For automatic transmission Mustangs, a Chip or Flash Tuner can adjust the speedometer for the gear you have installed.

Because ring and pinion gears require shimming and experience with special tools, a professional installation at a market rate of $200-250 is recommended. A rebuilt posi-trac or limited slip differential is a great thing to have done while installing gears because it will only cost the price of a rebuild kit or parts since the rear end is already opened. Auburn, Eaton, FRPP, and Detroit Locker make quality closed differentials for Ford Mustangs, with Auburn being the highest quality. V6 Mustangs have 7.5″ open differentials stock, and all Modular engine GT, Bullitt, Mach 1, and Cobra Mustangs have an 8.8″ posi-trac differential stock.

All of this information is true for N/A Mustangs. The supercharged (S/C), turbocharged (T/C), or nitrous Mustangs may need smaller gears than the 4.10 to trap at the right RPM in the 1/4 mile. There are calculation tables here to find out what gear is best for your application.

Now that you are informed about the functions of ring and pinion gear sets, and now that the many speculated misconceptions have been undisputedly disproved for N/A cars, go get yourself some 4.10:1 gears. If you have a built rear end with stronger axles, buckle up and get some 4.56:1 gears. Don’t fear the gear!