Take two cyclists: Cyclist A can sustain a maximum power output of 250W while Cyclist B can only manage 225W. On a perfectly flat, smooth indoor track (where gravity is not an issue) we can confidently predict that A will be faster than B. On an undulating road, however, power-to-weight begins to matter more. If both cyclists weigh 80kg, A will still be faster. But if A weighs 80kg and B weighs 68kg, cyclist A’s power-to-weight ratio is 3.13W/kg, while B’s is 3.31W/kg. On a flat road, there might not be much in it, but head into the hills and it is cyclist B who will be pulling away.