Don't think of it as only getting the fuel economy of an ordinary car based on your driving pattern -- because that's wrong. For as poorly as you think your HEV is doing, an ordinary SUV (or an ordinary car) would be doing just that much worse still.

My *other* car is an Audi A6 4.2. It's rated mileage is in the low 20s in the city and mid 20s on the highway. But... when it's cold outside and the car hasn't warmed up, fuel economy is in the LOW TEENS and my instant fuel-economy reads SINGLE DIGIT VALUES when I accelerate (I am NOT a lead-foot -- that's gentle accelration of a very cold engine).

I'm guessing that if your FEH is giving you 18, a comparable non hybrid would be giving you about 12 mpg.

The economy of the FEH is NOT just based on the battery system. The engine fires on the atkinson cycle. It produces a little less HP, but it also consumes a lot less gas to do it. Overall it provides much better economy than a traditional engine. Even when you're on gasoline, you're still burning less fuel than a traditional car.

You can read more about here:

http://www.answers.com/topic/atkinson-cycle

Or just type "atkinson cycle" into Google.