what speedo gear?

for your car you actually need a 21 tooth gear. The 23 tooth gear if for 90-93(maybe more) with the 8 tooth drive gear. The 85-89 t5 came with a 7 tooth drive gear, therefore needing the 21 tooth.
 
Driven Gear = Drive Gear x Rear Gear x Tire Revolutions Per Mile¹ / 1000

20.89 = 7 x 3.73 x 800 / 1000
21.28 = 7 x 3.73 x 815 / 1000

¹Factory tires are at 800-815 RPM. Aftermarket tires (even with the same specs as factory tires) are not necessarily at the same RPM. Check with the tire manufacturer.
 
Yes, he was correct that in 90 they changed the drive gear from 7 to 8 tooth on the T-5. I just got my gears done On friday and just went threw this. With a 89 and earlier stang with a T-5 and STOCK wheel diameter, you want a 21 tooth speedo gear. Since you don't have a stock size tire, you may need a different gear use the link that jrichker gave you.

Mario