A serial / series hybrid is much more efficient than an ICE -- because the electric motor is so much more efficient than the ICE. It is about 85-90% vs 5-20%. And the clutch and transmission only lower the ICE's efficiency.
Any serial hybrid should be a plug-in (heck any hybrid should be!) and the other efficiencies of an electric car apply: you can get 25-45% of your energy regenerated in city driving; and somewhat less than this on highway driving. There are no idling loses. There are no warm up time (for the electric motor).
The serial hybrid motor has a fixed load, at a fixed torque. It can be much smaller because the peak torque is what the generator requires, and it only has to keep up with the average demand on the battery. And so, the engine can/should be tuned to peak efficiency at that known output.
We have several known examples of serial hybrids: diesel/electric trains are serial hybrids -- I wonder how it is they can move a ton well over 400 miles on just one gallon of diesel? The other example is the early
Mini hybrid: it had four 160HP hub motors (for a staggering 640HP total!) and it only needed a 250cc ICE to charge it's batteries -- and it got ~80mpg in charging mode.
I can only imagine how much better FE it could get if it "only" had four 40HP electric hub motors -- 160HP from electric motors and four wheel drive, traction control, regenerative braking are all simple to implement -- this Mini hybrid has 'em.