check your o2 sensor
check your cat
check your plugs

but to put it in perspective....

on a good country run, i can get from adelaide to ballarat (maybe a little further) on a single tank of fuel (babying it, using cruise control, keeping it under 2000rpm)

but by the same token, if i go through a stint where i am constantly caught in slow stop-n-go traffic, where my accelerator and brake both get a hefty workout and you pray for any opportunity to overtake, and make usage of the 'mystery third lane', i can use a full tank in under 250kms