Weird speedometer problem

i find it best to have your speedo read just a tad too high. Mine reads 2--3 mph too fast...so when I slipped up the other day and was going mid 70's in a 70 zone, the cop drove off as i went by but then turned around at the first turn around.

U remember it for a while but then as month-years ago by, you forget about it. Why I did not see the semi marked car sitting on the shoulder, I don't know, my brain is slowing down too much....