[mustangtestuser]

Do not trust the stock tire rev per mile. My original SCT tune was way off after my gear change. I couldn't figure out what it was until I did a little testing. It turns out the "stock" rev per mile was off. I was probably driving around going under the speed limit the entire time I had the tune! When I changed gears I started examining my speed more closely to make sure all was well, which was the only reason I discovered the problem.

When I got my Bama Chips tune I manually entered the rev per mile to avoid any possible problems.

I eventually found out that the stock speedometer would read 3 mph high at highway speeds compared to what the computer was reading. I think the needle is a little off from Ford. No big deal. It just shows the hassle I went through to get all this corrected.

I'd do some followup testing with a GPS. Just remember that your speedometer isn't accurate at low speeds and the GPS is only accurate at best at stable speeds.

If you don't know your rev per mile, then you may want to start at 770 that will get you pretty close if your tires are the same diameter as stock. Check the manufacturer first for the number, then try Tire Rack.