if you rip out an engene from a car, then you can operate it at its most efficient rpm all the time, instead of when you happen to cross the right rpm on the right gear. So an engine powering a generator can almost always be more efficient than in a car.

This is not taking the losses in the electrical system when using the generated power in a moter to make the car move. But in a lot of cases, you still come out ahead

The West Australian Electric Car Club had to install a diesel genset part of the way across the country

That diesel was used more efficiently than in a VW golf in km/litre terms because generators are much more efficient than vehicle engines