<rest snipped>
Causes of poor fuel consumption include (in no particular order):
1) Failed / failing lambda sensor (also known as oxygen sensor). See
formatting link
2) Incorrect tyre pressures
3) Failed thermostat - does the engine get up to operating temperature OK?4) Failed temperature sensor - if it gets to the right temperature, does the engine managament system know it?
5) Binding brakes - do any of the wheels feel far hotter than they should? Does the car "Lurch" to a halt, or gently glide to a stop?6) Fuel leak - should be obvious by the smell! (But I guess if the computer is showing poor economy this is not the case for you..)
and finally, and most obviously
7) Forgetting to remove roof-rack, trailer or caravan ;-)Ian