the omega is not fuel friendly, its nearly 2 tonnes i beleive and any v6 will be thirsty. Add an auto box and busy town driveing with short journeys and 16 mpg is easy. Saintly driveing might give you 19/20 in town. Try using the auto box tendency to roll with no engine breaking to your advantage, ie gentle acceleration and come off the gas pedal early so you roll up to junctions or what ever with minimal breaking. Hard acceleration with long periods on the gas with hard breaking at the other end can waist fuel for very little time gain. Its a different driveing disciplin, try it, saves your license as well.
More fuel friendly cars exist obviously but youll have to spend alot more money up front to get one. Thus the total saving will take considerably longer to achieve/get your money back, probably not within the life of a smaller car. Personallly i would rather pay the fuel bill and have a cheaper more luxurious car.
The omega will go on, with some maintenance work, to do silly miles. With that in mind, a diy £750 lpg kit conversion can deliver overall savings within a year at about 50 to 55p a litre. The following 5 or so years, dep. on your mileage, of motoring will roughly half your fuel bills once the price of the kit has been recovered.
I would recommend a tech 2 session with one of the guys on here who have them. See "The Boys" rates on the bottom of one of his posts. This can check the multi ram operation on the engine and fuel trim(?) to make sure the economy is as good as it should be. Dont bother with a dealer tech 2 session, they ve not a clue how to use it. Muppets!
Hth.