Let's say that we have a mid-1950s "gas guzzler". Now, we take away the exterior "shell"/chassis and replace it with a modern car's. That is, it will visually look just like a modern car from the outside, but it's still using the exact same engine and technology inside.

Now, would this "improved" 1950s car drink less gasoline just because it has a different, "streamlined" shape? Or is this just a myth? Or is it technically true, but so insignificant that they might as well have kept on making beautiful/unique car models instead of making them all look the same?

I'm no physics/airflow expert, but it's always struck me as strange to hear how much the shape allegedly matters. Something tells me that what really matters is 70 years of continuously improved gasoline engines, further motivated by the increasing petrol prices.

Am I way off or close to the truth? I just have a very hard time "buying" that the shape could matter so much. It seems to me that if somebody in the car has had a hamburger recently, that extra weight in their body will more than make up for any "car shape-based gas-saving benefits".

  • $\begingroup$ Lots of books and references cover this. Check or research "frontal surface area" and rolling friction compared to air resistance. $\endgroup$
    – Solar Mike
    May 2 '20 at 21:12
  • $\begingroup$ Short answer, yes. Modern tyres alone would make more difference than your hamburger, too. $\endgroup$ May 2 '20 at 21:51
  • $\begingroup$ At highway speeds, aerodynamics is likely the largest factor. $\endgroup$
    – Eric S
    May 2 '20 at 22:03
  • 2
    $\begingroup$ For comparison, the human powered vehicle speed record (over a 200m distance on a level road) is just under 90 mph and the fastest downhill race speed is close to 150 mph. Compare that with an unstreamlined conventional bike and rider. Aerodynamics is everything. $\endgroup$
    – alephzero
    May 2 '20 at 22:28

The difference is in the drag coefficient.

Typical drag coefficient of the 50's cars have been in the range of 0.38 to 0.42 or so.

typical drag coefficient of modern cars even the ones not renowned for their streamlined body is ranging from 0.26 to 0.34.

drag is the retarding force exerted on moving bodies by the media they are moving in. it tries to slow the movement.

Some "high performance" models may actually have higher drag, due to wider tires and extra spoilers.

$ F_d=\frac{\rho C_dV^2A}{2} $

  • $F_d$ is drag force N

  • $\rho$ is air density 1.2 kg/m3

  • Cd is the drag coefficient. no units

  • V is the speed m/s

  • A projected area m**2

It is known that roughly for every 0.01 reduction in drag coefficient, $C_d$, an average care will gain a 0.2 mpg in average city and highway driving.

So 3 or 4 percent reduction in drag coefficient would be equivalent to 0.8 to 1 mpg fuel saving.

Modern cars have much more efficient engines and many other features that makes them more efficient but that is not the scope of this question.

  • $\begingroup$ I was always surprised that the smaller cars didn't quote drag rather than drag coefficient. None of them quoted projected area so it was meaningless marketing. $\endgroup$
    – Transistor
    May 3 '20 at 14:16

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