I used a GPS app to get true speed on a straight level road where I could hold speed. Speedo said 60; GPS said 47. I have repeated this test at different speeds, always getting the same percentage error. I know the differential ratio because it is stamped on the ring gear. I could get a more accurate tire diameter by using the rollout method, but I am off by a lot more than measurement error of the tire diameter. Taking the speedometer out of my restored IP would be a real pain and would probably scratch up some new paint. I will drive it for a while and if it is stable, I will probably get a 38 tooth gear.