Project MILF 2006 Ford Mustang GT Gear Install - Fueling Around ... "You fax us information with your new gear ratio and we overnight you a new calibrator," he says. ... In the following chart, ...

If your Mustang has a 3.35 First gear and a 3.55 rear gear, the overall First-gear ratio is 3.35 x 3.55 = 11.89. Anything greater than 9.5 could be considered overkill for the street.