After reading the post regarding the Diablosport chip. I would pay the extra bucks and purchase the chip. You can make the gear change as well as many others from the Diablosport chip.
If you buy a hypertech programmer you can reprogram the speedo and as well set shift points, tire sizes, set rev limit, set firmness of each shift and much more. (do your research)
In short the speed cal intercept the signal comming from the transmission and send it back to the computer with the right ratio. From what i have seen the put it under the shifter panel.
For my i bought a diablo predator. The only down side is you can't only change the ratio (if you want to have a clear conscience ). But you can change the ratio when you put the tune in.
I have a speedcal and it is a little box that has dipswitches in it. You mount it under the shifter panel. You can adjust the speedcal to different tire sizes too. It works for all gear ratio's too. Look at the dallas mustang web site or better yet call them .