Your average single-engine piston gets ~22mpg with a single passenger. It's not terrible at all.
22 mpg is not terrible only if your frame of reference is pickup trucks, whose load-carrying capabilities can only be matched by far larger aircraft with far higher fuel usage.
Furthermore, aircraft that cruise at 250 knots are not going to get anywhere near 22mpg. A Lancair Evolution, a reasonably modern four-seater turboprop that cruises at around that speed gets something approximating 7 mpg.
I personally don't see the point of comparing a car at 50mph with a small plane at 120mph, but if you really want to, I bet that easily doubles fuel efficiency. No one flies at Vg because that's stupid, but you COULD, and then you're basically the same fuel efficiency as a car but flying in a straight line.
Even that 22mpg is not a 1:1 comparison because of that inconvenient "as the crow flies" thing. We can build more efficient aircraft in theory, but in practice the regulation gets too expensive.
Long EZ reference: https://generalaviationnews.com/2016/05/25/going-fast-on-les...