Check with the GA DMV now GA DDS. I believe calibrated speedometer is required by law. If the dealership sold you the truck without the speedo calibrated then I would skip the pawns and go straight to the General Manager. Quote the law and they should fix your issue. If not let them know you will be reporting them to the BBB and anyone else that can help you. It only takes 10 minutes to recalibrate the speedo.
I installed 285's on mine last night, called the dealer and was told GM will only allow them to calibrate for 265's and it would take two hours labor at $85 per hour. What a rip!!!!! So it's either buy a programmer and use it anytime you change tire sizes or pay the dealer for 2 hours labor for a 10 minute job. For fun tell them you want to watch as they calibrate. I would be nice to see what they do with the other hour and 50 minutes .
First, I'm stationed in CA right now and as we all know, everything is jacked up in CA!!
Second, I though about doing exactly what you said, but as I have EFI on the way anyway, I decided it wasn't worth the effort to argue with them. Its irritating, but I don't want to piss them off now, in case I actually need some work later that they might have a legitimate excuse to refuse.