real effect of head wind and tail wind on mpg
 

I read a message somewhere, where the poster concluded that the mpg while driving at (1) 60mph with a 10mph tail wind will give the same mpg as driving at (2) 40 mph with a 10mph head wind. His conclusion was probably based on the fact that in both cases the wind was flowing past the car at 50 mph as both scenarios cause the same effective air drag on the car. (To use aircraft analogies, air speed is 50 mph in both cases, the ground speed differs)

Now I would say the car in scenario (1) above is probably getting almost 50% higher mpg than (2) above because it's covering 1.5 times the distance with the same effective air drag. This improvement in mpg is far and above what on might expect by driving at 60 instead of 40mph with the wind scenarios above. Also for a more typical American car (automatic with larger engine), the car in (1) will be more likely in its engine sweet spot with regard to BSFC.

Personally, I definitely see the improvement in mpg with my 2008 2.4L Scion xB when there is a 10 mph tail wind, where I can average 45 mpg at 60 mph. Get this it goes down to 40 mpg when I reduce the speed to 55 mph, so going faster up to a point when you have a good tail wind is good for the pocket book. Going against the 10 mph wind going the other way I get 20 mpg at 60 mph, a reduction beyond what one might expect.

So I have yet to determine what is the best speed with a 10 mph head wind or any other head wind speed for that matter. The same for a 10 mph (or other speed) tail wind, what is the best ground speed.

Any answers to this would be appreciated.

on my last trip to texas from CA, I went thru Tuson Az on Hwy 40 heading east.
At one point I went into neutral(just to see the effect) and it took a while to finally slow down. Got about 7-10% better mpg.
on my return trip (west) it cost me about 12-15% in mpg going into the wind.

Didnt really do anything scientific so I cant give any hard #s

...huh? I-40 goes thru Flagstaff, across northern Arizona; I-10 goes thru Tucson, across southern Arizona.

...the "physics" says differently: drag is a squared function of speed, while HP is a cubic function of speed...and MPG is basically a linear function of HP.

The answer will be specific to your vehicle, engine, and final drive ratio. So you, and only you, can answer your question.

HUMM along

"the answer my friend is blowing in the wind, the answer is blowing in the wind"

[QUOTE=Old Tele man;264594]...huh? I-40 goes thru Flagstaff, across northern Arizona; I-10 goes thru Tucson, across southern Arizona.

Yes....your right.....it was flagstaff............I feel soooooo :o

Not just head winds, side winds are pretty bad too. We've had north-south gales for most of this week and I head east-west and back again every work day and this has not been fun or given good MPG figures and no P&G.

wind
 

In the aero-mod sticky or aero seminar sticky there are some values from SAE research investigations on wind effects which have been published by the EPA.
In the book 'More with Less' there is a discussion about the GM/AeroVironment strategy for wind during the International Solar Challenge back in 1987.
Low power vehicles will suffer greater losses in wind.
I just recently got 47.9 mpg at 65 mph with the 4,800-lb T-100/trailer combo in a 26-mph tailwind in Colorado.About 22.9 mpg better than its baseline for the 'naked' truck.
My CRX could see a 50% reduction in hwy mpg based on wind conditions.
Things might need to be taken on a case-specific basis regarding this topic.

This has been my experience, as well.

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.

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.

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

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

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.

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.

EPA: wind effects
 

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.

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.