Fox Body Rear Gears & Ratios Explained

• Gear end ratio is the ratio of input revolutions to wheel revolutions
• Differential gears do not change power levels, but rather, manipulate what power and torque is available at certain RPM
• Upgrading rear gears is dependent on the transmission gear ratio
• Choosing a rear gear is decided by the type of Mustang and the type of transmission

Before we get into the options and such, first let us take a remedial mechanics class and go through gear ratio operation. For reference, when talking about gear end ratio, it is the ratio of input (driveshaft) revolutions to wheel revolutions (i.e: 2.73:1 – 2.73 rotations of the driveshaft produces 1 wheel revolution).

Before choosing a gear, let us consider why you should change your Mustang's gear ratio. First and foremost, differential gears do not change power levels (you will not see an increase in engine HP or torque). They manipulate how much power and torque (mostly torque) is available to you at a certain RPM to help you either accelerate faster or have a higher top speed. By stepping down to a lower gear (a numerically higher ratio, i.e 3:1 is lower than 2:1), effectively you are raising engine RPM per wheel revolution. What does this mean? In a nutshell, a lower gear ratio (remember, a lower gear actually has a higher numerical value) will help you step off the line faster by manipulating torque. 'You hit the power band sweet spot in less time. The trade off? Well, because the engine is now turning a higher RPM for a lower overall speed, you are going to sacrifice top end speed because you’ll simply run out of RPM (this is also known as “running out of gear”).

I like to use a mountain analogy to explain this. Think of a high gear (lower numerical ratio) as a very tall, steep mountain, with the peak representing top speed. When you swap over to a lower gear (numerically higher ratio), imagine taking the peak off of the high gear mountain and adding it to the base of the low gear mountain.

Interpreting these ‘mountain’ diagrams, we can see with the high gear top speed is greater, engine RPM at a given speed is lower, and time given to reach a speed is higher. Examining the low gear diagram, we see overall top speed has been lopped off, engine RPM has increased per any given speed and subsequently time to reach a given speed is less. With that out of the way, let’s move on to choosing a gear!

As previously mentioned, choosing a gear ratio is not a black or white decision. It is highly subjective to each individual car, and how the owner intends to run that car. The first variable to take into account is your transmission. The automatic and manual transmissions Ford used over the years came with different transmission gear ratios (Oh joy, another gear set to worry about!). Here’s a quick breakdown:

As you can see, apart from the SROD, the other automatics all have a pretty low first gear ratio. Coupled with the factory rear end gear ranging from 2.73:1 to 3.27:1 depending on trim, the automatics are more get-up-and-go compared to the manual equipped cars. Even then, the manual equipped cars are leaving a lot of potential on the table with their high factory differential gear. The later generation of T5-equipped Foxes had a slight advantage over the earlier models, but again, a gear swap will really wake the beast up. What does this all mean? Well, quite simply, a higher first gear will need a lower rear-end gear to optimize it’s off the line performance.

When purchasing gears, you must know what rear end you have. Early Foxbody Mustangs came equipped with a 7.5” rear end, whereas 1986-on came with an 8.8” differential. The same ratios are available for either size differential; just make sure you pick the correct ring diameter for the rear end.

We’ve talked a lot about choosing a gear, but without actually talking about choosing a gear. As previously stated, the factory rear-end gear ranged from a high 2:73:1 to a slightly lower 3.08:1 and to the lowest 3.27:1, depending on the year and option package of the car. Regardless, any of these ratios aren’t so hot with a stock ride and leave a lot of acceleration to be had. The aftermarket has really (and I mean really) conceived every possible ratio to give some tire-burning low end acceleration. Anywhere from 3:27:1 to 4.56:1 (4.56:1 is REALLY steep) are available as a direct replacement. Despite the massive choice, luck has it that there are 3 preferred and proven ratios that stand out. The 3 ‘kings’, shall we say, are 3.55:1, 3.73:1 and 4.10:1. The aftermarket Mustang world has done much of the work for us, and it is safe to say that one of those 3 king gears are chosen in 95% of all applications. Which one to choose, well that is what we are about to weed out.​

As previously mentioned, the transmission gear plays a bit of a role in your decision. Referring to the higher geared (BUT numerically lower ratio) automatic transmissions, an automatic car typically needs more gear than a similarly built manual car. For this reason, the minimum gear most enthusiasts would recommend for an automatic is 3.73:1. In fact, the majority go one step further and bump up to 4.10:1 with very exhilarating results.

Word on the street has it that naturally aspirated manual cars are best off using either a 3.55:1 or 3.73:1 gear ratio. The three fifty fives are more suited to an all-around driver whereas the three seventy threes are considered to be more performance orientated. However if you look up 3.55 vs. 3.73, you’ll see there is an endless debate over these as well. 3.55’s offer plenty of shake and bake over the stock ratios, but most enthusiasts opt for the extra oomph provided by the 3.73’s. Neither ratio will leave you disappointed!

One final aspect to consider is the tire size you’ll be running. Believe it or not overall tire diameter does have an impact on gearing. Generally the hardcore race guys are the ones looking into this, (they want to be flying through the traps at redline, in the highest transmission gear) but it is something everyone ought to consider as well if you plan to run tall, drag slick tires.

The last thing worth mentioning about a gear swap is the effect it will have on the speedometer. All Fox era Mustangs utilize a mechanical pickup speedometer with a gear placed in the transmission for measurement. When changing out the stock gears, this stock speedo gear must be swapped for the appropriate one as well, otherwise both the speedometer and odometer will measure and read incorrectly.

Upon completion of a gear swap, you can expect stronger acceleration off of the line and better quarter mile times. Again, a gear swap does not increase or decrease actual engine output. It just manipulates what there is in the best way possible. For this reason, gears are considered to be a top bang-for-buck mod for all Mustangs.

Furthermore, it is possible to see an increase in fuel mileage puttering around the city due to the engine reaching and maintaining peak efficiency levels for longer durations. At highway speeds, you’ll turn a few more RPM than before and subsequently sip a little more fuel.