The Stovebolt Site Forums The Shop Area Driveline Speedometer inaccurate after R&P swap

I had the 4.11 in my '53 3100 converted to a 3.55 and, predictably, the speedo is now off. What's puzzling, though, is that it seems to read 6-7 mph slower than my actual speed (as measured by GPS) across all speed ranges. For example, when my speedo reads 20 mph my real speed is 26; when it reads 35 I'm going 41; when it reads 55 I'm doing 61, etc. This makes no sense to me as I'd expect a consistent ratio but different ranges at various speeds. What's up and how can I correct this. Thanks