SFC varies with throttle setting, altitude and climate. For jet engines, flight speed also has a significant effect upon SFC; SFC is roughly proportional to air speed (actually exhaust velocity), but speed along the ground is also proportional to air speed. Since work done is force times distance, mechanical power is force times speed. Thus, although the nominal SFC is a useful measure of fuel efficiency, it should be divided by speed to get a way to compare engines that fly at different speeds.
For example, [[Concorde]] cruised at Mach 2.05 with its engines giving an SFC of 1.195 lb/(lbf·h) (see below); this is equivalent to an SFC of 0.51 lb/(lbf·h) for an aircraft flying at Mach 0.85, which would be better than even modern engines; it was the world's most efficient jet engine.<ref>[http://www.pbs.org/wgbh/nova/transcripts/3203_concorde.html Supersonic Dream]</ref> However, Concorde ultimately has a heavier airframe, and due to being supersonic is less aerodynamically efficient, i.e., the [[lift to drag ratio]] is far lower. In general the total fuel burn of a complete aircraft is of far more importance to the customer.