There's actually a huge range in private planes. This M600 plane does about 7 mpg, 250 knots at 45 gph. A typical light jet might do 400 knots at 125 gph (3 mpg). But a light jet cannot not fly nonstop to a different continent. For these trips, a large business jet might do 450 knots at 300 gph (1.5 mpg), but only the very super rich can actually afford these. Even most celebrities will be flying the airlines for long international trips.

Thanks for the calculations! It’s weird how a more advance technology like a jet can be so successful yet an order of magnitude less economical. Usually it’s the reverse. But when you’re rich it’s time that’s money

The short answer is that higher speeds come at a steep price. The relationship between velocity and the power required to overcome drag at that velocity is cubic, so that large business jet requires on the order of 50x more power to fly at 450 kts than it would flying at 120 kts, like the little Cessna might.