Speedometer gears

Tire Circumference = Tire Section Width / 25.4 * (Tire Aspect Ratio / 100) * 2 + Wheel Diameter * Pi

Tire RPM = 63360 / Tire Circumference

Driven Gear = Drive Gear * Rear Gear * Tire RPM / 1000

Available Drive Gears: 6T, 7T, 8T
Available Driven Gears: 16T, 17T, 18T, 19T, 20T, 21T, 23T (not Ford)

'83-mid '89 T-5: 7T drive gear
mid '89-'95 T-5: 8T drive gear

If the transmission in this '95 GT is a T-5, the drive gear is 8T. Any gear ratio lower (higher numerically) than 3.55:1 there is no driven gear available for. (And even then, 23T driven gears aren't Ford gears, they're Chrysler gears and they're crap. The teeth are thin, and they will wear out quickly. There'll be plastic bits floating in your transmission and the speedometer will be bouncing all over the place. Expect to change 23T driven gears often.) You must switch the drive gear to 7T or, preferably, 6T and get the appropraite driven gear.

If the transmission in this '95 GT is an AODE, you have two problems. First problem is that the drive gear is machined to the output shaft. That means the whole shaft has to be changed. That means a transmission teardown and rebuild. The second problem is that the AODE is electronically controlled and it WILL NOT live a long and properous life with anything lower than 3.55:1 gears. Read THIS article.

EDIT: Corrected typos.