Speedo Gear Question

I've got a friend with a 95 mustang gt with a 5 speed transmission. We're trying to figure out which speedo gear to use. He has installed a 3.73 gear.

According to Steeda's website it looks like we need a white/23 tooth gear:
http://www.steeda.com/products/speedometer_recalibration_gears.php

According to this site, it wants us to use a 7 tooth drive gear and a 21 tooth driven gear or a 6 tooth drive gear and a 18 tooth driven gear. I also noticed this chart does not go past 21 tooth, so maybe they don't stock anything over 21 tooth so their chart will not suggest the 23 tooth that steeda is suggesting. Here is this link:
http://www.hanlonmotorsports.com/T5Speedo-3.asp?Width=245&AR=50&Rim=16&tdia=&Ratio=3.73

So basically i'm trying to figure out for sure which gear to use. Does anyone have 3.73 gears and what speedo gear are you useing? Any insight anyone has would be greatly appreciated.

Thanks.