Drivetrain Speedo issue

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.

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.