I know your combo is very similar to mine (4.62 gears and a C4). Do you recall what size speedo gear you had to use to correct your speedometer. I've got slightly shorter tires, but I'm just looking for a ballpark idea.
I tried a couple of different driven gears and gave up. I think the closest I got was about 10 or 15 miles per hour off. Its been so long now that I don't remember what color I left in there. I tried to find some kind of conversion formula that would tell what gears it should have but I never found one.