For what it's worth, I don't have the speed wire hooked up and I have pretty accurate speed displayed relying on the nav/ gps. I've contemplated hooking it up for the sake of it, but found it not necessary. This is on a 6100nex, and it's been in two different vehicles so far without speed wire.
Perhaps you need a factory reset? Pioneer recommends a reset after a fresh install. Not sure if you did that already or not, but it may or may not help. Being disconnected from that battery only resets so much, so a manual reset might be needed. Sure it may be a pain having to reconfigure everythi