Even when I use my HRM on my bike, I get a figure that's probably twice what I would expect.
Most of the time, I suspect I burn 300 - 400kcals an hour when cycling around here, or on a flattish audax.
Based on lots of dull analysis of food intake, weight loss, HRM and cycle-computer figures, various web-pages, GPS tracklogs and elevation data,
http://www.kreuzotter.de/english/espeed.htm and lots of handwaving, figures plucked from thin air and guesswork. I'd agree with that.
From what I can work out, my cycle computer (Cateye Velo 8) gives vaguely accurate figures (probably within 10% of the real consumption).
For my commute in (which I go at pretty hard and average about 28kph) it's about 500kcal/hour.
For an Audax (20-24 kph) it's 300-400 kcal/hour.
My Polar HRM gives kcal consumption figures pretty much almost double these, and therefore wrong.
But for running, the HRM kcal figure is reasonably accurate, but a little high, in that it fits with expected kcal consumption figures based on the treadmill speed and my weight.
I can maintain 160bpm on the bike for hours on end (indeed I averaged 160bpm for the first 200km Audax I did). But I can't maintain 160bpm whilst running for anywhere near as long.