So here is a breakdown of how well we did gas-wise on the trip.

Seattle to Albuquerque, including driving around Yellowstone, was 2063.4 miles.
Albuquerque to Seattle, via the Grand Canyon, was 1777.7 miles.
Total distance round trip was 3841.1 miles.

We made 14 stops for gas with an average distance of 274 miles between stops.
Average fill up was 7.89 gallons at 2.95 per gallon.

Total fuel used was 110.51 gallons at $326.36.
Average mpg was 34.76.
Average cost per mile was $0.08.

I am pretty happy with these numbers.  FIrst off, we (just barely) beat the high EPA estimate for mpg that the Civic is supposed to be able to get.  The mpg was understandably lower on the first day when we were climbing in elevation towards the Rocky Mountains and Yellowstone.  I was watching the ScanGuage and we were usually getting about 29-32 mpg realtime, and 30 mpg average per tank at fill ups. This is confirmed by looking at the average mpg for each leg manually (dividing each fill up in gallons by the distance between stops).  On the descent home I was hoping to make that up, but we must have had headwinds because I was only seeing about 32-36 mpg.  But we made excellent mpg during the middle legs of the trip.  Between Albuquerque and Salt Lake City we were making 40 mpg averages for whole tanks.  Between fill ups at Flagstaff and Fredonia Arizona we averaged 43 mpg for the tank! This wasn’t just coasting down hills in neutral (I can get over 300 mpg doing that for short distances)… this was hundreds of miles at 43 mpg! We must have had a tailwind or something, because it wasn’t all downhill driving… it was relatively flat.  My estimate for carbon usage was pretty close too. NativeEnergy.org calculates 1.081 tons of carbon from driving 3841.1 miles at 34.76 miles per gallon.  That is half of what it would have been for the two of us to fly to Albuquerque and back, and is almost exactly what I estimated last month.

Cost savings, energy savings, 7 days through 9 states… all around great trip!