Standard IC engines in are of low efficiency because they need to work at a variety of speeds and loads. Engines that run generators can run at a constant speed and thus can have valve timing and profiling, etc., optimized to that specified speed and load.
The IC engine in a hybrid car can be quite efficient, more than enough so to make up for the loss of efficiency of the generator and electric drive motor(s). The gas-electric hybrid was invented early in the last century by Ferdinand Porsche and was the principle behind the diesel locomotive. Hybrid vehicles have the advantage of good pick up due to the good torque found in electric motors. They should catch on.
Hybrid cars are innately expensive to manufacture because you are manufacturing two motors, each capable of driving the car. Also, unless a battery, with its high replacement costs, is part of the system as a power reservoir, you are still forcing the primary engine to run at fractional power almost all of the time. Also, you still haven't gotten beyond the innate inefficiency of the primary gasoline engine. The hybrid is a costly interim solution with only marginal economy advantages.
If fuel availability gets to be a drastic problem, the first answer will be 70 mpg cars that are very lightweight, very aerodynamically efficient, and have very small gasoline or Diesel engines and barely adequate performance by today's standards. Think of a modern "deux cheveaux".