I'm a little confused. I assume that this acquaintance is NOT actually a policeman?
Even if that's the case, I still think he wasn't necessarily rude for giving you a "heads up" that you were speeding (assuming you were in fact, speeding!). I don't want to sound preachy, but speed is a huge factor in many accidents, and if you're exceeding the speed limit, that can endanger other drivers in the vicinity (like your acquaintance).
Provided he did so in a polite discreet manner, and not a rude, public manner, I don't think he was rude at all for remarking that you were speeding.