Theoretically, even if never plugged in, they should get much better MPG due to the regenerative braking and the ability to always run the combustion engine at the most efficient revs (which for city driving should ~halve fuel consumption alone).
The reality is due to a series of poor design choices, that's rarely the case. Normally due to certifications on the braking system, the regen braking isn't actually hooked up to the brake pedal, so most braking is done with the regular friction brakes. The engine also doesn't normally run at the most efficient RPM because users don't want to hear the sound of the engine revving while they're stationary. Combine that with the fact there are efficiency losses in the battery and electric motors and the added weight of the battery, and you suddenly have a system worse than a regular car.
One of the main reasons to go all electric, maybe there’s some design slight of hand that can be used to increase the time electric power is used.