This was last summer in the 28' Pantera with 502's right before I sold her...
Full load of fuel, a $hit ton of beer, 6 people on board, and the drives weren't all the way up- I figure with a skeleton crew, 1/3 of a tank of gas and, most importantly, no beer ballast, she'd do about 82.
That's not a GPS speedo but we checked it against a GPS unit and it was just about spot on.