How did I miss this the first time round?
+1 (or +3 or whatever) for calibration. I did a very rare tempo session on a gym treadmill recently and realised something wasn't right when I'd cranked it up to "18kph" but was jogging along with heart rate still down at 60% max. I could've "done a Rushton", stayed on the treadmill to impress the gym bunnies for a bit, then come on here to brag about it, but I had some proper training to do, so I got on the treddy next to it and knocked out 8 miles at 16.6kph instead*. Obviously I have no way of knowing how accurate this was, except to report that my heart rate was about the same as I would expect running at the same pace outdoors, and I was sweating like a pig in cling film.
*...and then came on here to brag about it. 
Edited: 19/03/2013 at 21:05