real effect of head wind and tail wind on mpg

[puddleglum]

Originally Posted by Arragonis

Not just head winds, side winds are pretty bad too.

A partial side wind can even be worse than a direct headwind because it is increasing the effective frontal area of the vehicle as well as the wind speed. There are some other threads which discuss this quite well.

[briank]

Thanks for the feedback so far but I do have a question for jakobnev

Originally Posted by jakobnev

The force acting on the car is the same, but since speed is 50% grater in one example you need 50% more power! (P=V*F)

But the energy is the same, it's simply force times distance: W=F*D

So it's all down to what power lets the engine reach the better position on it's BSFC-chart.

jakobnev:
For the P=V*F formula, shouldn't the speed V be considered as the effective air speed not the ground speed. Here's my logic with this. So if you had a 50 mph tail wind and this helped sustain the car along at 30 mph without any engine power, and you then turned on the engine and just pushed the gas pedal ever so slightly to sustain 30.1 mph would it be incorrect to write this as P=30.1*F ?, would it be more correct to have something like P=0.1*F, with most of the power likely overcoming the rolling resistance (F). I suspect operators of chassis dynamometers would know a thing or two about the mpg at various speeds with the absence of air drag.

[jakobnev]

It would be P=30.1*F, but with very low F.

[JRMichler]

Here's some data to play with. Today I drove straight west 80 miles against a direct headwind. The wind was 15 MPH gusting to 25, temperature 45 degrees, elevation same at both ends. Stayed long enough for the engine to cool off, then returned. Same route, same wind, temperature 48 degrees F. Same speed both ways - 55 MPH. No drafting either direction.

31.0 MPG going, 38.2 MPG return.

Engine efficiency was lower on the return, with much of the trip at only 6 to 7 PSI MAP. One more gear in the transmission would have been nice.

[t vago]

Indeed. I also travelled against a pretty strong headwind going into work today (WUnderGround.com says it was around 19 MPH coming from the west). I can normally get around 22 MPG coming into work. Today, I could be lucky I got 20.2 MPG. I figured that the truck encountered airspeeds of up to 90 MPH.

[aerohead]

According to EPA,the following effects were measured for a passenger car traveling at 50-mph in an 18-mph wind
* headwind= 17% mpg loss
* crosswind= 2% mpg loss
* tailwind = 19% mpg gain

[KamperBob]

Originally Posted by aerohead

According to EPA,the following effects were measured for a passenger car traveling at 50-mph in an 18-mph wind
* headwind= 17% mpg loss
* crosswind= 2% mpg loss
* tailwind = 19% mpg gain

Interesting data - thanks.

Note: the gain and loss symmetry. -17% is 0.83x and its inverse, 1/0.83 equals 1.205x or +20.5 percent. Interesting indeed.

[briank]

Interesting to see how much these EPA findings can be applied amongst the wide variety of car shapes. I've heard of some cars being more aerodynamic in reverse, which is not hard to believe because most people prefer a "pointy front design" to their cars.