Originally posted by RobG: Presumably the figures quoted in Wikipedia are "average".
Actually those figures typically come from whatever the manufacturer posts as a design goal tied to a mission specification intended to satisfy a given commercial market; there's no standard configuration for comparisons. No two designs will ever have best/optimum fuel/range/payload/seat capacity in common.
If the four engine, Airbus 340 gives up a fuel economy advantage to a wide-body twin it may be to off-set expensive ETOPS constraints on trans-oceanic legs, say between NYC and Johanneburg, SA.
The factors at play in choosing a commercial aircraft are fascinating -- pick a route structure, a payload (250-475+ seats?) and a required range (9000- to 19000 KM?). There's a whole world of difference in the numbers when you factor in a mission statement and the variable trade-offs in payload, fuel economy, route structure, predominate weather, etc. E.g., sometimes you give up economical fuel burn to increase speed to get that last available gate time at a hub. Or to squeeze in an extra trip per day on a popular route where assured full seats offsets fuel cost.
As a MUCH simpler example, in my 7KCAB with 36 gallons of no-reserve fuel the POH says I can go 390 sm @130 mph @ 2500' MSL burning 11.6 gph in 3.0 hrs OR 815 sm @90 mph @10,000' MSL burning 3.8 gph in 8.5 hrs OR most anything in between those numbers. However, there's only 540 lbs of total discretionary payload to be divided between two seats and fuel which weighs 6 lb/gal. (In practical terms, however, my personal 'holding tank' is only good for about 3 hrs max in any case which tends to 'define the mission plan!)