I had a similar positive bike-car interaction a while ago. I was riding in the left lane, since I had to make a left turn, and a car came up on my right and was pacing me. He rolled down his window and shouted "25 miles an hour!" I was flabbergasted but said something short and friendly back (no sense in alienating a driver!) When we stopped at the light, he kept his window down and chatted with me, talking about how he thought bikes belonged on the road with cars rather than on sidewalks (!!). We continued the discussion at the next light, where I turned left. The encounter had me smiling all day. At least some drivers understand.