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.