I've seen everything now!
The truck arrived a few days ago dripping fuel out of the tank onto the bed of the rollback, I could see what looked like a puncture. By the time it was unloaded the tank was empty otherwise I'd never had known the tank had a leak and would've pulled the bed to replace the sending unit.
Pure luck it was leaking.
I crawled under to find a pinhole in a brandnew steel tank, I cleaned around the hole and saw a blueish halo around the hole. When the muffler guy welded the muffles (they run right under the tank)he missed with the mig wire and ran it into the tank! That was what shorted the other sending unit! The tank didn't leak until it was filled completely after it left my shop.
I patched the hole and just put some gas in it and the gage reads proper without leaks!
Pure luck that it leaked when they delivered it, if all the fuel had run out i'd never have known about the hole and would've pulled the bed off, removed a perfectly good sending unit and would've been totally dumbfounded.
