Yeah, and within the constraints SAE J1226 that most US manufacturers follow in terms of speedo accuracy, there's 4% tolerance (though specific speed biased). 31 or 32 in a 30? A judge would laugh that out of a courtroom and tell the officer to focus on more important tasks.

And anymore in 2017, you pretty much need documented, calibrated instrumentation (even "pacing" doesn't fly anymore).
Bottom line, they had no idea he was speeding, he looked to be going "about the posted limit", it was late, his tags were funky. They didn't really care about his speed, it was just an excuse for a stop, because so many good arrests result from this sort of police activity.
FWIW, I don't go over +5 MPH on the highway (in 65-70MPH posted), and around town I creep, very slowly, because my car is obnoxious and loud