The newest planes are the most efficient, and, in general, bigger is usually more efficient on a seat-miles per gallon basis. The data is filed with the DOT using "Form 41" and is available to the public. There are various places that break the data down into usable info, such as ALPA's Economic and Financial Analysis department, Aviation Week, and MITs Airline Data Project. I'm biased, but ALPA's E&FA is the best of the bunch, staffed by a group of excellent economists and other analysts.
He had to run to go and fly a 777 from San Francisco to Frankfurt (he'll be in Germany for the German team's World Cup semi-final, cool!) but he did leave me with some tantalizing data:
Here's some fuel burns for a few of yesterday's flights. I don't have time to look up seating capacities or mileage because I'm just about to leave to fly SFO-FRA, but I think you can find out this info for yourself.
Flight Route A/C fuel burn (gallons)
900 SFO-FRA 747 34,000
926 SFO-FRA 777 22,400
914 SFO-IAD 767 7,460
120 SFO-IAD A320 4,180
In general United has fewer seats per aircraft than most other airlines, because of Economy Plus and because of the number of first and business class seats.
So united gets a knock for having fewer seats on planes. Southwest and low-cost air carriers get a bonus for that but they fly smaller planes, so how does it work out for this one set of observations?
Well SFO to FRA is about 5700 miles and SFO to IAD is about 2450. United's 747 has about 374 seats, the 777 has about 258 for international travel, the 767 about 244 and the A320 about 144. So assuming all seats are filled, which is not accurate, but these days a pretty good approximation the seal miles per gallon for the actual flights from yesterday are as follows:
900 SFO-FRA 747 34,000: Seat miles per gallon = 62.7
926 SFO-FRA 777 22,400: Seat miles per gallon = 65.7
914 SFO-IAD 767 7,460: Seat miles per gallon = 80.1
120 SFO-IAD A320 4,180: Seat miles per gallon = 84.4
Oops, contrary to the conventional wisdom shared above, the smallest aircraft, the A320, did the best. Followed by the next smallest and so on. So here smaller is better (though don't take this too far, little commuter jets are terribly inefficient). Actual loads, weather, winds, delays, etc. could all effect the fuel burn. The 747 for example, is supposed to do closer to 90 seat miles per gallon.
Note that flying beats driving almost surely (unless you have a relatively fuel efficient car with many passengers), especially when you consider how many extra miles you have to drive relative to flying. [And for you smart-alecs out there, yes, I am referring only to the SF - DC flights]
Maybe I'll get him to give me some more burns when he gets back so I can look into other types of flights.
In the future, I'll try and flush this out a bit more and see if I can generalize about companies, aircraft, etc. Because, you know, what else do I have to do?