question about 93 5.0, 5spd with upgraded gears

Your speedo may be off 10mph at 70 mph. And at 10 mph your speedometer may be off 2 mph. Remember, the difference of speed is actually a percentage. So your speedometer will read an actual percentage of increase. For example if your car initially had 3.08 gears and you went to 3.73 gears. That is a 21 percent change. So your speedometer will read 21 percent higher than actual speed. The change in speed depends on actual speed.

To find out what gears you have jack the car up and support it on jack stands. Keep it in nuetral so you can rotate the driveshaft. Mark the drive shaft in relation to the axle housing. Also, mark each wheel in relation to the backing plate. Next rotate the pinion and count how many turns of the driveshaft it takes until the wheel marks rotate on revolution. If it rotates 3 and 3/34 turns it is a 3.73. If it rotates a little past 3 1/2 it is 3.55 gears and so on.

Lastly, there is not a speedo gear that will work with 3.73 without changing the output shaft drive gear. This requires you to disassemble the transmission. Most guys use a 23 tooth which is correct for a 3.55 gear. This will get you close using 3.73s. Also, the 23 tooth gear tends to wear out because the mesh with the output shaft gear is not perfect. The best solution is the replace the output shaft and drive gear as a pair. However, most guys with 3.73s just throw in the 23 tooth and are close. Your speedo will read approx. 5 percent higher than actual speed with the 23 tooth output gear. If you ever have the trans apart it would be a good idea to replace the drive gear while you are in there.

This thread has more info on the speedo gear.
http://forums.stangnet.com/showthread.php?t=687147&highlight=91notchbk