So you were over the posted speed limit by 6mph, even over the normal "buffer of 5mph". Your registration was 3 months expired, not just by a couple days. He "did you a favor" and didn't cite you for speed. Maybe he was bored that night. I don't know what traffic or road conditions were, what else had been going on that night, what he saw about your car, etc. Later at night the chances of drugs, OVI, etc go up so for him to pop a u-ey in the road for a registration out by a whole 3 months isn't that bad. I don't know all the details but with the information you gave me I can't find fault in the officer's actions. Next time renew your registration on time because I doubt he would have pulled you over for speed alone, although he may have.