A good diesel generator should have the peak efficiency of a diesel engine of about 40%. A diesel car can hit the same peak, but only at a certain speed/power output. So the average efficiency is about 20%. So there is a quite a marging for charging losses.

The weight of an electric car usually is a minor component in the consumption, most is the aerodynamic drag. On top of that, electric cars can get about 60% of their kinetic energy back by using their motor as a generator.