That's about right, though there are a few additional considerations:
1. Aircraft have heaviest fuel use during takeoff and ascent. Short flights are the least efficient. I'm not aware of a good modeling for this, but you might consider that there's a fixed takeoff/landing/taxi cycle and a variable cruise component to most flights. Also, larger aircraft (at capacity) tend to be more efficient than smaller ones, though that's a fairly varyable dynamic.
2. Automobiles offer numerous other capaibilities (and limitations). Generally, more cargo, and more direct transit. With airport delays adding 1-3 hours to a flight, that's 150 miles / 240 km of ground travel possible at highway speeds.
3. More luggage capacity / fewer restrictions. You can carry items on a car you couldn't on an airplane.
4. Point-to-point service. Aircraft fly to airports. Cars drive to specific addresses. That's another consideration in on-ground time, possibly several hours, in a flight. Autos start looking viable for trips of 200-250 mi / 320 - 350 km.
5. There are larger autos availabe. Five adults in a small compact might not be reasonable, but might consider an SUV or van. And efficient choice here would make up for lower fuel economy with higher passenger capacity.
Aircraft still generally win on safety on a pro-rated distance basis.