Thanks all for your comments. Winds must have shifted this past weekend because mileage was slightly better going south than going north. I do not know actual mileage because I used one tank for both directions. We will try again this weekend and see what reults we get.

Don't forget also that elevation isn't the be all end all of a hills effect on MPG.... Trying this with ascii art.... say you have this hill:

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MAN since they kill spaces on this darn forum it sure is hard to do any ASCII art Sheesh

Going left to right you are probably going to burn more fuel than right to left, but its complicated. Physics says the same amount of "work" is involved in raising the car up then down again (actually the net "work" vertically is zero if I remember my physics right but that's another matter), however, the car is going to have to be in a lot lower "gear" (high RPM for higher torque) to climb the steep hillside than to climb the shallow one going the other way and high RPM's aren't good for gas mileage.... On the other hand you are going to coast a long way down the other side, getting some of that back... now if you go left to right you are probably going to have the brakes on (unless you have a lot of guts) and use loose energy that way. I THINK left to right will be worse for MPG-- but there is a lot involved. Even when regenerative braking hybrids have losses in braking, the energy is stored in the battery, but there is still some lost to heat---its a lot better than loosing all of it.

In any case MPG differences in round trips for each direction aren't that surprising.... there are a lot more reasons behind it than there seems on the surface. Just because you start and end at the same elevation doesn't tell you everything.

I agree with Whiterook and TeeSter that it is the elevation much more so than the wind. You would be amazed at how you think a road is flat, but there is a very slight incline, which raises RPMs and reduces MPG. Look at your tach often each way, and I think you will find your RPMs will be more often near 1500 one way than the other. Prior to buying my 2005 FEH 4WD, the only vehicle that I displayed estimated fuel economy was a 1997 Ford Crown Victoria that my parents gave me. On the couple of occassions that I drove it to work -- each time requiring filling up with gas near home --, it would indicate 21-MPG on the way to work and 17-MPG when I got home, meaning it got 13-MPG on my way home (an 8-MPG difference!).

Altitude can make a difference but not in your case. 23 feet is nothing over 150 miles. Temperature does make a difference but once again this is not making much of a difference in your case.

The wind is almost definitely the reason for the difference in mileage. Your trip north is really north and east which is the general direction for prevailing wind.

I notice a similar effect on round trips. On a typical busniess trip I travel west and generally get lower mileage fighting the headwind. The return trip is eastward and is typically with the wind so mileage usually better coming home.