Is there an equation or way to calculate how much the speedometer is off when the rear end ratio is changed?

 

easiest way to tell how far the speedo is off is to get a friend to go with you on the highway and have him run a a steady speed like 55 mph and then see what your speedo reads. you could do this at several different speeds to get some idea of how far it is off. I know when I changed my 2.73 rear gear to a 3.73 rear gear, the speedo read 85 mph when I was doing 55 mph until I changed the speedo gear in the tranny.