Alas i can't remember and a quick web search doesn't find anything I recognize. But I did find this that mentions 10hp at 50mph: http://energy.typepad.com/the-energy-blog/2010/02/energy-nee... And a few hotrodders discussing the question here: http://www.hotrodders.com/forum/how-much-hp-required-60mph-1... This on PhysicsForums: https://www.physicsforums.com/threads/power-and-tq-needed-to...

And finally a paper with real formulas and better explanations: http://wps.aw.com/wps/media/objects/877/898586/topics/topic0...

Looks like my 5kW wasn't too far off.