Mark,
Call Jim at PATC (Performance Automotive and Transmission Center) 318-742-7754.

I called him and he had me go out, using either a GPS or an iphone App and note the speeds at 30, 40, 50 & 60 of both the truck's speedo and the GPS. According to my iphone ap, the truck was reading about 21% low, he had a 20% speed up adapter in stock. Close enough for me. Works perfectly and very accurate. A whopping $62 smackers. Takes PayPal or credit card.

Craig

My '50 Chevy 3100 5 window, '62-235cu, 3:55 rear
My truck ....... Respect The Rust
If I'm not working on my truck, '65 m00stang or VW camper, I'm fishing with the wife or smoking Salmon.