We’ve talked a lot about choosing a gear, but without actually talking about choosing a gear. That’s a lot of talking and not much choosing so on to the options. 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 transmission 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, as they’re usually referred to, are more suited to an all-around driver whereas the three seventy threes are considered to be slightly 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. Whether we’re splitting hairs discussing 3.55 vs. 3.73, the only way to find out is to try ‘em out! Nonetheless, neither ratio will leave you disappointed!
One final aspect to consider and help you come to a final decision is the tire size you’ll be running. Believe it or not, overall tire diameter does have an impact on the gearing. Now, it is generally the hardcore race guys that really look 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 there is plan to run some seriously tall rubbers.