How do you drive it? Have you done any mods like exhaust or air filter which make a great sound when pushed but obviously MPG will suffer. So drive it carefully but enjoy ablat once in a while on an empty open road.
When was it last serviced?
1, Is the thermostat correctly opening at the right temperature.
2, Are the wheel bearings in good condition if they drag it can cause a lot of friction
3, Tyre pressure - this can have a big effect on fuel economy taking upto 6 mpg if the pressure is wrong.
4, has someone attached a caravan or trailer to the back without you noticing.
5, Are the local kids syphoning off the Fuel overnight/do you have a fuel leak.