Updating Speedometer

[zibzer]

Last year I upgraded my 2000 Silverado 4.3l rear end to a 3.73 Posi unit (FYI awesome upgrade but would go 4.10 if I could do it again) . Ive been driving around with the wrong speedometer readings for a year because I havent been able to find out a way to fix it. The dealer didnt know how to do it, an independent shop didnt know how to do it, and the programmers I have looked at don't work for the 4.3l engine. anybody have any suggestions?