Tires: tire sizes vs. spedo, tire failure, radar gun

Question
I just bought a 2005 ram 4x4 quad cab slt.I felt as if the spedo was off and
had it tested with a radar gun and confirmed that the vehicle was traveling
slower than the spedo read.The tires on the vehicle are p255/65/r17 however
the manufacture sticker on the door reads that there should be p265/70/r17
on the vehicle.would this be the reason there is about a 2mph difference at
40 mph on the speedo and nearly 4mph difference at 65 mph on the spedo? I
look forward to your answer.

Answer
Jim,

http://www.1010tires.com/TireSizeCalculator.asp

I used this calculator, and while tires do not have to meet a certain diameter regulation and could be made to any diameter - this calculator works pretty well in estimations.

It says that there ought to be a 6.4% error which at 60 mph would be a 4 mph error, which at little smaller than what you got.

BTW, The tire that is on there now is also smaller in load carrying capacity - and that could lead to a load related tire failure, which sometimes has tragic consequences.

I'd recommend you get tires of the placard size on as soon as possible.