Check your wire connections. For a 1997 why did you waste your money on a Speed Cal? All you need is the little nylon gear that pops into the trans. If your connections are okay, check all the little switches to make sure they are set right. If they are, then you need a new Speed Cal. They are junk. I went through four of those before I demanded a refund. Two of them did the same thing yours is doing. I nver got the Speed Cal to work right. I bought a tuner instead to change my ratio.
Just get the speedo gear. They come in different teeth sizes. Like 16,17,18,etc. I am not 100% sure what size you need for 4.10's, Look on eBay.
http://cgi.ebay.com/ebaymotors/83-98-Mustang-23-tooth-3-73-4-10-Speedo-Gear-T5-T45_W0QQitemZ260079592183QQihZ016QQcategoryZ33731QQssPageNameZWDVWQQrdZ1QQcmdZViewItem