Guys, thanks for the response.Yes, a 50 mpg would be a huge shock.

Yes,NO is at sealevel +- 4-5 ft.Flagstaff is about 6900 ft, so it is uphill.The 1500 was one way.I left a few things out to make it simpler,since it is still 2 problems-1)Scanguage reads close to 20% too optimistic; 2)My MPG is about 2-4 less than it should be.
The other factors
1)Yes it is uphill, but it was uphill in Aug(evac) when it got 24mpg on the last 1000 miles(DFW to Flagstaff).It would have been even lower than the average since the worst tank was a 15 mpg from Alb-Gallup.
2)I move the scanguage from car to car-Pilot,Prizm(Corolla),Prius. I don't "tell" the scanguage I'm doing this-meaning I don't do any adj.;I just pull it out of one and plug it in the other.Heck, it is just taking fuel flow numbers and speed numbers and dividing.How tough is that?It is as if it is either misreading fuel flow, or I am losing gasoline??
3)My airfilter was old(36000 miles).This could hurt actual mpg, but it shouldn't hurt the Scanguages ability to read and divide?Same story on spark plugs,PCV valve etc.I've changed the airfilter,and will change the pcv-probably won't bother with the plugs.
4)I have done this trip many times in 4 different vehicles.The east to west leg is always about 1.5 miles less than W-E.A tiny bit of this is the 7000 ft drop.I'm fairly sure that prevailing winds across AZ,NM,TX are the major factor in the 1.5 mpg difference.
5) I have oversized tires(long story which could-probably do-but not 100% sure they hurt hy mpg).I have corrected for the size.I had these same tires on for the return trip in Sept.It was 24mpg(corrected) on the return trip in Sept 2005.
6)I'm not so sure the prevailing winds in Aug Sept are the same as they are in early June.
Well,what do you think.The poor mpg certainly could be because of dirty filter,bigger tires,unlucky winds,and a bit of slosh evaporation.. Could the switching of the scanguage have "thrown it off?".I confess,I really didn't spend much time reading the scanguage manual.
Brick-I didn't think the evav system vented to the outside? The tank has an overflow for liquids, but I didn't think there was anyway for vapors to escape?
On the trip home I'll have a clean filter,and will stop filling at the 1st "stop".I think I was losing fuel; maybe the evac system/slosh.I was stopping at the 1st stop last trip.I didn't do it this time because we were driving at night in remote areas.I was concerned with range.I added at least 1 gallon after the 1st stop.(8 fillups-we stop fairly often-walk dog etc)
Thanks,Charlie
PS I gave you corrected mpg.The distance is 1495 miles(many trips,same route,different cars-all read within 8 miles of 1495)-my trip meter read 1418-about a 5.4% difference.It is exactly what I expected with 245/75 x16 vs 235/70 x16- roughly .75" taller. I used ~76 gallons.