There isn't a gear you can run to calibrate for 3.73s, all SN95s with VSS use an 8 tooth drive gear and the most you can calibrate for with a Ford speedo gear is 3.27s which is the purple 21 tooth you have. You actually should have had a grey 18 tooth gear for 2.73s but w/e. Anyway, you'll need to get a tune in the computer or a speedcal. The 97 trans uses two sensors, an OSS for the shifting and a VSS for the speedo which is why your car is still shifting ok.
You do not have to drop the trans to do a speedo gear anyway, its right on the left side of the tailshaft but you don't need that. Since you have a 21 tooth purple gear in there your computer thinks you have 3.27s. Buy a Dallas Speedcal, install it and then use their chart to calibrate it for 3.73s with your starting gears as 3.27.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.