Also, as a rule of thumb, fuel consumption per unit distance is proportional to speed squared. According to the chart, the "Double Bubble" cruises at 490 MPH,

Also, as a rule of thumb, fuel consumption per unit distance is proportional to speed squared. According to the chart, the "Double Bubble" cruises at 490 MPH, while the 737 cruises at 596 MPH. The "Double Bubble" achieves 145 MPG, while the 737 achieves 100 MPG. (490/596)^2 * 145 MPG) = 98 MPG 737s are already on the slow end of airliners, but it seems like all the fuel savings in this design study come from just flying slower.