Having problems finding right speedo gear

[1995GTStang]

I just swapped to 3.73 from 2.73 gears. The place that did the work put a black 20t speedo gear into the T45 transmission. At 60mph my speedo was saying I was doing 71-73mph. I swapped out the 20t for a red 21t speedo gear and now at 60mph its says I am doing closer to 65mph. Has any one else had this issue at all? I am not sure what is offered past 21t but it appears to me that I may need a 22t gear. The speedometer was pretty damn close to being right on before the gear swap.

Any ideas?

 

[LaserSVT]

What size tires? You should be closer then 5mph with that gear, should only be off 1-2 MPH.

What you can try is changing the drive gear from a 7 tooth to a 6 tooth and then putting the 20 tooth driven gear in it. The part number for the 6t drive gear is E3ZZ-B.

Also what are you using to check the speed against? A GPS unit or someone elses car?