its pretty easy to fry the guage itself, I burned up one, all I did was cut the stupid wire(when the key was on....doh!) with a pair of snips, after that the guage was WAY off, swaped in another and all was fine.
not sure if thats it or not but seeing how you just rewired it could be.
It is easy to out the wire on the wrong contact point and fry the gauge. I had this problem from the PO or his mechanic, and the gauge was constantly over the top. Put in a new gauge, and set it up on the right contact and it works fine now.