I'm not an expert, but I believe it gets its biggest advantage in stop and go where it recovers a part of the energy thru regenerative braking, but at speed what economy it does have is no different than any super-small car would achieve with an undersize gasoline-powered engine, except it's slightly worse with the (in that scenario) dead-weight penalty of the batteries. That's my cursory analysis as an engineer going thru a mental visualization exercise.
Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')