I once had a different differential put in a truck and the speed was off on the speedometer due to a changed ratio... So I went to a "speedometer repair shop" (large cities have these) and they made me a "custom" reduction box (connected between cables) and that made the speedometer accurate.

I got the impression they could do anything speedometer related, even a bit of magic! So may want to see if your city has a speedometer shop or maybe in a larger city near you.