Yeah, that seems to be really convenient. My 05 Tundra is a bit more of a pain in the ass since Toyota has a habit of locking down their ECUs. I have larger tires on my truck as well and I guess I just have to deal with it. Most people install some speedo corrector in the wiring but I hate the idea of installing something additional to take care of what I should be able to adjust myself. I can understand locking down the data that deals with driveability and such but they should know that people like me want a "Man Truck" and are going to customize their trucks to look tougher. Just make it easy on us to change tire sizes!
Anyway, yesterday I discovered something really cool as far as speed inputs are concerned. Like I said earlier, to display the correct speed in MPH, you need to modify the stock sending unit from 20-pulse to 12-pulse. Thankfully, Ford has been using a sensor that does exactly that! The 1994-1998 Mustangs use a VSS on the side of their transmission that ouputs the 12-pulse per revolution signal we need. On top of that, they are dirt cheap! On Rockauto, I'm seeing new ones for under $15. The part number is SC37T. This is what it looks like:
Now, there is a small issue. This is a VR-style sensor. It puts out an AC sine wave. The dash needs a 5v square wave to function properly, so for that you need to make or buy a VR-conditioner circuit. I would buy the Brick VR conditioner/
because it is small and also because it runs on 5 volts. This will convert the AC signal to a more friendly square wave signal for the dash. The convenient thing about running this speed sensor arrangement is that you no longer need to run the speedo cable all the way up and around to behind the dash. You can just run a few shielded wires down to the sensor near the transmission. This is environmentally sealed since it was designed to be used on the transmission. It will handle most anything you dish out.