Q&A

# Mailbag: Choosing a Ring-and-Pinion for a Fox-Body Mustang

(Image/Hot Rod)

Q: I have a 1991 Ford Mustang LX couple with a bone-stock 302.

I’ve always thought about getting an 8.8-inch ring and pinion set for my Ford, but I’m having difficulty picking one out. There are so many choices in ratios, but I can’t quite get a grip on what they all mean—can you help me tell the difference between them?

A: Before sinking your teeth into a rear-gear swap, it’s important to know how gear ratios will affect your Mustang’s driving behavior and overall performance.

Rear gear ratios are determined by dividing the number of teeth on the ring gear by the number of teeth on the pinion. For example, a 43-tooth ring gear divided by a 10-tooth pinion gear yields a ratio of 4.30:1, or, the pinion rotates 4.3 times for every one rotation of the ring gear.

Once you have your ring-and-pinion’s ratio, multiply it by any of your transmission’s gear ratios to determine the final drive ratio in that gear—lower final drive ratios provide lower cruising rpm and slower acceleration, while higher ratios will make for quicker takeoffs at the expense of greater rpm at highway speeds (which also increases engine wear).

For a stock Mustang, a lower-ratio ring-and-pinion, like one of these 3.55 Mustang gear combos will provide a good all-around performance boost without taking away from everyday streetability.

If you’re planning on taking your pony to the track, a higher-ratio gear set would be a better fit.

1. G Simpson says:

A lot of Mustang’s came stock with any where from 3.07:1 to 3.27:1. Make a chalk line on the inside of one of your tires. Mark the yoke ( companion flange) at the axle. Rotate the drive shaft and count the number of times it takes to bring your chalk mark back to the start point. If there’s not an axle ratio tag this method will work to give you a rough idea of what axle ratio you have. 3.55:1 is good. 3.73:1 will give you a little more punch. 4.11:1 probably a little steep, but loads of punch off the line. Don’t forget to replace the speedometer gear in the transmission. The old calibration excuse never really works.

2. Brian says:

2.73 and 3.08 were the most used ratios for Foxbodys. If you have an auto , go 4.10. If its a manual, 3.73

3. HAROLD LOVETT says:

Unless you are going to take it to the drag strip every weekend, go with the 3.55 gear. It is a lot of fun and will give you all the performance your stock engine and trans can handle. While you have the drive shaft out is a good time to replace the old U-Joints. Don’t forget the Ford friction additive plus synthetic 75-90 gear oil.

4. Daniel Wilson says:

Another great reply from the experienced Staff at OAC for a question that is on the minds of literally thousands of proud 5.0 liter Mustang owners seeking a fairly quick and cost effective means of achieving seat of the pants rapid acceleration on or off the strip.

Naturally the level of fuel consumption will increase accordingly, especially with the steeper (numerically higher) ring and pinion ratios like 4.11 or 4.56:1 and more due to the overall increase of engine rpm’s. But it has always been that way and those are the dues we pay for quicker play !

Many years ago I bought a used (and abused) complete Ford 9” center section loaded with 4.56:1 gears and an early Ford Equa-Loc clutch type limited-slip differential for a reasonable price. I installed it behind a totally stock 351C with its 2-V (barrel) heads and 2- barrel carburetor. Yes indeed it accelerated quicker but not what I expected and less than spectacular. The totally stock low compression Cleveland would have come alive with a few well chosen performance parts and then the Drag Racing specific differential gear ratio would have come on like a madman all the way up to its reduced 100 mph top end speed. Before the gear ratio change, 125 was normal…but took forever with the 2.79:1 granny gear ratio. LOL !

Pretty tasty nuggets of trial and error for anyone considering a new performance gear ratio.

And the beat goes on…..

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