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The fastest fighter aircraft is the MiG 25, with speeds topping at Mach 3.2 (2,450 mph/4,000 km/h). Pilots were not allowed to go beyond Mach 2.8. Above Mach 2.8, the engines would overheat and burn up.
Assuming that you have a magically infinite fuel source, it really depends on the plane. The F-86 Sabre (D variant) could go 698 mph, faster than a 747. That means that it would be possible to travel around the world in about 35.67 hours. However, another plane easily beats that time. And it is beautiful. The SR-71 Blackbird has set the current airspeed record at 2,193.19 mph (3,529.6 km/h).