Speedo gear in the transmission... just pull the speedometer cable out and there it is. It's a little plastic gear- you'll need to replace that with a another color. I need the same one so hopefully someone will tell us what color we need
If found this site that sells gears and speedo driven gears. It appears the same speedo driven gear works for both the 3.73 and 4.10 We can buy it at the link.
with 3.73's you might still not be able to get the speed 100% correct. I believe 89 and older you can get the correct speedo gear but 90 and up they changed and you can only correct up to 3.55 rear. 50resto will tell ya.