Concorde had considerable difficulties that led to its dismal sales performance. Costs had spiralled during development to more than six times the original projections, arriving at a unit cost of £23 million in 1977 (equivalent to £152.02 million in 2021).[37] Its sonic boom made travelling supersonically over land impossible without causing complaints from citizens.[38] World events had also dampened Concorde sales prospects, the 1973–74 stock market crash and the 1973 oil crisis had made many airlines cautious about aircraft with high fuel consumption rates; and new wide-body aircraft, such as the Boeing 747, had recently made subsonic aircraft significantly more efficient and presented a low-risk option for airlines.[39] While carrying a full load, Concorde achieved 15.8 passenger miles per gallon of fuel, while the Boeing 707 reached 33.3 pm/g, the Boeing 747 46.4 pm/g, and the McDonnell Douglas DC-10 53.6 pm/g.[40] An emerging trend in the industry in favour of cheaper airline tickets had also caused airlines such as Qantas to question Concorde's market suitability.[41]