I fired up SportTracks this morning and looked at some Activity Reports. The cycling data covers both fun rides from work with mates on my mountain bike, commuting on various bikes, and going out on my road bike.
And average speed second. These are all on my Orbea Orca - I can't go this fast on a mountain bike.
I'm pleased with the 20mph average over 15.57 miles. It is a route that avoids steep hills. Average heart rate over 46.8 minutes was (only) 157bpm. (In 2007 I averaged 177bpm for 32:07 whilst running 4.15 miles, 7:44 pace). In the graph you can see the correlation between gradient, speed and heartrate.