a strange thing happened on the way to work

I've had a week of mixed luck on the bike this week. Tuesday saw me pick up a puncture at the midway point, Wednesday was wet, and today I had a car driver shout at me.
Normally a motorist shouting at a cyclist is as a result of the driver thinking they own the road (well they do pay road tax...), but today it was because of something completely different. I sensed I had a car right on my back wheel while going down a nice little downhill stretch, but there was also a hint of a following wind. I'd noticed I'd touched 52kph (about 33mph) on my speedo, but the driver was actually shouting at me to tell me I'd clocked about 37mph. Of course by him slip streaming me, I got a marginal advantage of about 3-4% on top of my own efforts!
I'll be taking the train tomorrow, so nothing can go wrong...