Tire height and gear ratio are inputs to the PCM and used to control the electronic transmission (4L60 or 4L80, etc) The ratios and heights requested by the tuner only have to be close for it to work just fine
If you're using a 350, 700 or a manual transmission it isn't needed, at least in theory. My truck is a 5.3L with a manual transmission which has no speed signal (VSS) from the transmission, and it will sometimes drop the idle very low when I put it in neutral and coast to stop (the VSS trims the IAC). I'm told Dakota Digital has a speed signal generator that solves this problem, but I've not experienced enough issues to investigate it.
