ditto what Matt posted
http://www.oldchevytrucks.com/cart/product.asp?prodid=SP249&i=9931|||||"1932-1972 Speedometer Ratio Adapter Fits all GM Applications GMC Pickup Truck
Item Number: SP249
Special Order item, No Returns. Must Pay In Advance!
Retail: $97.50 EACH
On Backorder [added by tclederman: custom made to match your needs - see below]
Corrects the error in your speedometer when your rear tires or a total differential has been changed. Fits all GM applications. Simply follow a vehicle with a correct speedometer at 50mph for one mile and tell us the mph your speedometer reads. We will custom build a ratio adapter for your truck. Takes minutes to install to your transmission! Made in the USA. No Refunds"Note: you could also use a MPH/speedometer/GPS app in you cell phone to obtain your truck's speedometer reading when the MPH/speedometer/GPS is reading/displaying 50 mph.
I have used these types of adapters and they are excellent -- if speed is measured correctly, and if the ratio adapter is manufactured correctly.
