I have an Avic F700bt with the bypass. The speed sense wire on the harness is strictly used for what Pioneer calls dead reckoning. The headunit uses the signal from the speed sense wire plus the gyroscope to attempt to keep track of your position when you lose GPS signal. For example when you go into a tunnel or a parking garage. Truth be told, it doesn't work too well in a parking garage after you're gone up a few floors. The gyroscope just thinks you're going in circles.
