Ok, I had 29 inch tires on my truck, I just put 31 inch tires on my truck, how much is my speedometer off exactly? I know I could go out and have a buddy drive next to me at varying speeds, but I want to know first what "should" be happening.
You now have larger tires, so now when your axle rotates you will move farther. So your speedometer will be a tad slow. 29 to 31 inches should'nt make much of a difference. If i had to guess i'd say 3mph difference at 55mph. The faster you drive the more its gonna be off though. Someone correct me if im wrong though.
My roommate, (who is a lot better at math than me) agrees with you. It sounds about right to me, because on the interstate when the speed limit is 65 I pass most people that are in the right lane.