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

The difference in the speeds is due to the speedometer head itself. The needle is moved by a magnet. The magnet is cable driven. As the speeds increase, the small spring attached to the needle is over come by the magnetic pull and the needle moves. Over time, the spring and/or magnet loose power and the speedometers ability to maintain an accurate reading goes away.
Figure out where you are going to drive the most, say in town or on the highway, then have the speedometer calibrated to that speed. Anything above or below may be off, but at your chosen speed it will be accurate.

Most any decent speedometer shop can build a adaptor to compensate for the gear change. They may even have new speedometer gears for the transmission, depending on what you have.

Your odometer is gear driven right off the cable, so it will read the same no matter what speeds you drive. If you can drive a ten mile stretch, you will be able to figure out the amount of error. This along with your GPS information will be enough for the adaptor to be built.

Joe

That makes sense, Joe. Thanks

Denny, if you do the math, the conversion should have given you a 13.6% increase actual speed versus speedo reading making the assumption that your speedo was dead accurate before the change. Since you are now benchmarking it off GPS, it is hard to tell what your true increase was versus the speedo unless you have the same GPS/speedo data points from before the installation.

The fix is simple as Joe points out. I bought an adapter from Jim Carter and am very happy with it. His website has instructions of what and how to measure to get your adapter custom made for your application. As I recall, the unit is calibrated for accuracy at 50MPH knowing that there will be some deviation above and below that. Other companies also offer an adapter. Here is a link to Jim Carter's.

http://www.oldchevytrucks.com/cart/product.asp?prodid=SP249&i=|speedometer%20adapter||||

Matt

Thanks, Matt. The adapter calibrated for 50 mph should do the trick.