Drivetrain Speedo issue


New Member
Aug 31, 2022
Hi everyone I appreciate any help I can get I have a 1993 five speed 5 L mustang with a T5 with 273 gears in the rear end the stock speedo gear should be a 18 tooth but when I put an 18 tooth in it the speedo reads 20 over so I put a 23 tooth in it and it reads 10 over I’m not sure why this is happening does anyone know how to fix this also I have the stock ponies with the stock tires size thanks for your time

  • Sponsors (?)

I don't have my spreadsheets in front of me, but 2 things could be contributing here, other than mechanical problems. First, it's possible that the drive gear in your T5 is a 6, 7, or 8. Don't just assume it's the original T5 drive gear, unless you know for sure.

2nd, the speedometer needle itself can be installed wrong. Usually, folks who pull out the cluster and mess with it will point it to 0, when it should actually be pointed at a mark below that, in order for it to be calibrated correctly.
I think stock gear with 2.73s is a 16-tooth drive gear.

You should have a green/white 8-tooth drive gear inside the t5.

The fact that even the 23 tooth gear has the speedo reading high makes me wonder if the speedometer is reading correctly. Can you compare with GPS to see if the speed is dead on? Can you also reset your trip odometer and do a gps app and see when you pass 1 mile and see if the tripometer matches as well.
  • Useful
Reactions: 1 user
Same thing I wrote over on the Corral:

I am betting that you do not have 2.73's. If you are still 10 mph fast and you are running an 8 tooth drive gear and a 23 tooth driven gear this means you need more teeth on the driven gear.

Jack the rear end up (use jack stands to support) and see if you have an open differential or a trac lok unit (posi). If your car has an open differential lower one tire to the ground if it has a posi then leave it like it is. Turn the tire 10 turns and count the number of turns of the driveshaft makes. Divide the number of driveshaft turns by the number of tire turns and that will get you the rear gear ratio.

Examples would be:

35 1/2 drive shaft turns / 10 is 3.55
27 1/3 drive shaft turns / 10 is 2.73

When you know for sure what rear gears you have then you can determine if you need to change the drive gear on the output shaft of the T5.

I have 3.73's and run the black 6 tooth drive gear as it allows me to use a much lower driven gear tooth count which means it will last a long time. The 21 and 23 tooth driven gears seem to strip pretty easy.

You may be able to see the color of the drive gear on the output shaft of the T5 by pulling the speedo cable and removing the VSS. Use a mirror and if you see a translucent (jacked up yellow greenish color) yellow gear its the 8 tooth. If it is yellow then its the 7 tooth. I am betting its the 8 as 90 thru 93 cars came with them stock.