I went to Dallas over the past weekend in the new Prius .. along with my wife.

We had A/C on (temperatures were very hot, and the A/c was blasting off all along the way) .. and we were doing about 70 miles per hour. The MPG reading at the end of the trip to Dallas was about 53 MPG.

On the way back, we seem to have gotten pretty bad head wind, although I could not see the wind on the trees that were passing by us, or any other indication confirming the head wind, but the car seem to have been making a lot of swishing noise, as if it were cutting through a lot of wind. Also, the car was swaying and I had to keep a strong handle on the wheel ... I got 42 MPG on the way back!! OUCH!!

Can the MPG numbers really wary? Was it really head wind that we were getting? And if it were, can it be that bad??

Regards,

Sure, wind can cause a huge difference in mpg-so can rain. I'm surprised you couldn't see the trees move, but sometimes they just stay "bent in the wind" rather than sway.
I have also noticed that the Prius is a bit squirrely in wind; not quite dangerous, but noticeably less stable in wind of any sort.The mpg and the "funny feel' probably mean exactly what you think-head wind. Luck.Charlie
PS I've have driven thru your panhandle many times in other vehicles-talk about wind!!! I kinda wonder how the Prius will like that kind of wind!!

Yes a little wind can make a big difference. A 10 mile head wind is nearly the same as driving 10 miles per hour faster, a tail wind 10 miles slower. That's a 20 miles per hour difference one way vs. the other.

I have a 5 mile stretch due east going to work and I see my scangauge climb steadily from around 20mpg to 30+mpg during this stretch. On the way home it is either steady or drops 1-2 mpg on this same level stretch. It's all because of the wind though you wouldn't think it was "windy".

For some reason a cross wind seems to hurts the FE a lot too?

Yes, the wind effect is a vector. If you draw a straight line of say 4 units, for 40 miles per hour and then a cross wind of 3 units, for 30 miles per hour, the effective wind speed is the hypotenuse, 5 units or 50 miles per hour. In aviation we call it the wind triangle.

http://en.wikipedia.org/wiki/Wind_triangle

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

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I like the 5-12-13 triangle better.