I just bought a 1989 mustang GT and I was told that it had 3.73 gears in it. The speedometer is off by 20 mph. I looked online and they said I need a new speedo gear. Which gear do I need for a manual transmission?

 

On your T5, unbolt the speeding cable and gently pull it away and swap that gear for the right one. Check out Latemodel Restoration for the difference in gears.

 

Assuming it does have 3.73s and the tires are close to stock height I believe you'd need a 21 tooth driven gear.

OEM P/N C4OZ-17271-A.

-Steve