Well, a month ago I had 3.73 gears installed. Took my car to a Ford dealer that said they could correct the speedo. Well, they couldn't but they changed the tire size in the computer to reduce the error but the speedo still read about 7 mph high. I used a ratio calculator program and went by the RPM to figure this. I bought a Speed Cal and spent most of the day putting it in and using two different calculator programs I finally got the speedo dead on. The Dallas Mustang Speed Cal is great. It's not quite as easy to install as I thought but not really too hard. The last part was the hardest, figuring out how to stow both the Speed Cal and another module, a Trac Adapt and still get the console piece back in but finally I did it. The Speed Cal is a terrific product. 
