real effect of head wind and tail wind on mpg

[briank]

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.

[mcrews]

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

[gone-ot]

...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.

[JRMichler]

Originally Posted by briank

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.

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

[mcrews]

Originally Posted by JRMichler

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"

[mcrews]

[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

[Arragonis]

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.

[aerohead]

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.

[Mustang Dave]

Originally Posted by Arragonis

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

This has been my experience, as well.

[jakobnev]

Originally Posted by briank

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.

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.