Air resistance

Discussion in 'Gen 2 Prius Main Forum' started by labumm, Dec 2, 2004.

  1. labumm

    labumm Junior Member

    Nov 29, 2004
    2004 Prius
    Here's another tech question for the masses.

    I'm interested to know the effect of air speed on the fuel economy. Here in OK it is windy much of the time, so air speed can be much higher that the ground speed. It is a big effect because the drag is proportional to the square of the air speed. Has anyone seen a graph, equation, or quantitative discussion of this? It would be interesting to see plots of mpg fs ground speed for various head wind and tail wind speeds. This should show the "sweet spot" and give an indication of how much it shifts with head/tail wind.

    I figure it has been calculated. At least there must be wind tunnel measurements...

    It would be nice to have a pitot tube on this thing.
  2. amped

    amped Senior Member

    Apr 15, 2004
    Columbia River Gorge, Oregon
    Other Hybrid
    "But Officer, my airspeed indicator said I was only going 60."

    I recall a ground speed vs. fuel burn in no wind/headwind/tailwind conditions graph as part of the intro press kit. Too bad I can't find it right now. I've actually tried measuring the effect while driving through the Columbia River Gorge on my tire testing rides. The upwind leg of about 25 miles was on cruise control set at 65 into winds about 30 gusting 45. The immediate downwind return leg at the same set speed showed around a 10 mpg difference. I know that's a rough estimate because of random gusts, but I found it interesting that a major change in wind speed and direction did relatively little to consumption. Credit what I think is still the industry's lowest Cd.
  3. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
    Staff Member

    May 11, 2004
    Far-North Chicagoland
    2017 Prius Prime
    Prime Advanced
    Here's what I found with a quick Googling:

    [img=right:8a76e15289][/IMG]Motor vehicle fuel economies reveal the effects of environmental drag forces on enthalpies . Car buyers should be wary of laboratory tests claiming static fuel economies . Static test figures refer to a vehicle's unloaded fuel economy , as if travelling with a tailwind of the same speed , a situation presenting zero air resistance and the lowest possible load on the engine . In normal operation , with increased speed , the increased wind resistance produces an increase in fuel consumption , as illustrated in the graph to the right . These effects only become visible when the driver maintains accurate logs of fuel consumption between full tanks of fuel at each destination , recording distances travelled , driving conditions and terrain , loads carried , relative wind direction and average speed .

    Check the whole thing here: