Mach number
Mach number expresses an aircraft's speed as a fraction of the speed of sound at its altitude. Most business jets cruise between Mach 0.75 and 0.85; the Citation X+ and Gulfstream G650/G700 can reach Mach 0.925 to 0.935, the fastest civil aircraft flying today. Because fuel burn rises sharply near an aircraft's maximum Mach, operators usually plan long flights at a slower long-range cruise speed. For a passenger the difference between Mach 0.80 and 0.90 on a New York to London flight is about 30 minutes; on a one-hour hop it is negligible, so pay for speed only when the sector is long enough to feel it.