I did several runs in the late 1980s on the daily American 767 London to Los Angeles, 5,500 miles, which always seemed quite a haul for one.

Similarly on an American 767 London to Miami, which although about 4,500 miles direct took a notably longwinded route, passing close to Iceland and overhead Labrador and New York City, probably an extra 1,000 miles. Could be due to winds, but equally likely would be some Etops unserviceability which required the aircraft to keep within 1 hour's flying time of an airport.

Like other aircraft, there is plenty of space in the tanks for fuel, it's just that maybe at such extremes you need to leave 10 tons of revenue freight off in order to load an extra 10 tons of fuel. For the same reason aircraft like all-freight 747Fs may be fully loaded up with cargo but can only do say 3,000 miles of range at a time, landing with minimum fuel but at maximum landing weight. On routes like Tokyo to New York the passenger services nowadays are nonstop but the cargo operators, with equivalent aircraft types, invariably stop halfway along at Anchorage to refuel.