I know motor pacing can be dangerous so with that aside I have a question. On my ride this morning I was poking along (low heart rate zone) at about 17-19 mph about half way through my intended ride time got behind a school bus and drafted for a few miles at about 30+ mph stayed in my low heartrate zone (the upper end). I had my zone set on my heart rate monitor. After I lost the bus I was able to maintain 19-21 mph at the same heart rate (previous to drafting) as before for the remainder of my ride. Why is this? All conditions (road, wind etc.) were the same as before the drafting/motor pacing.