Not sure but I think the 3.90 gear was used in 60s era Fords with the 9" rear end (like the Boss 302). I don't think this rear set is available for the 87-93 generation Mustang with the 8.8" rear.
For the 87-93 year Mustangs the speedo is set accurately (supposedly) by changing the drive gear in the speedo cable where it meets the transmission housing. These speedo gears are refered to by the number of teeth on them. I think the 23 tooth gear is for the 3.73s.
If you're saying that reinstalling the old speedo without a gear change still resulted in 5 mph error, you've got something else going on there. don't know.
