[Ecky]

So, the reason hybrid systems gain efficiency and save gas are these:

1) You replace a low efficiency alternator with a higher efficiency generator, potentially saving several mpg
2) You can capture some of your braking or slowing-down energy, which can be used to accelerate your car again, greatly improving city economy, saving your brake pads and reducing engine wear
3) You don't need to rev your engine up as much when accelerating, because you produce more power down low, which usually saves fuel and reduces engine wear. You can even gear it taller, further saving gas.
4) You get rid of unreliable alternators and starter motors, and replace them with a single more efficient device that basically will never fail

In addition to these, you *also* get a huge boost in off-the-line acceleration from the added electric torque down low, making the vehicle more fun to drive.

To use my car again as an example, my engine is a 1L 3 cylinder with an 18,000w electric motor bolted to it. It's geared like a V6 or V8, because of its torque - I cruise on the highway around 1700rpm. When I need to pass someone, the electric motor gives me the passing power and I don't have to rev the already smaller, fuel sipping engine up to get that power, which saves gas AND reduces engine wear. Then, over the next few miles, it trickle charges the battery back up, taking a tiny percent of my cruising horsepower to slowly recharge the battery power I blew from 10 seconds of acceleration. I'm able to see 90-110mpg at highway speeds while still having a very drivable and fun car. It's no V8, mind you, but neither does it feel like any other 3 cylinder on the road.