Worst I ever did on a tank was 13.8 mpg towing a 1200 lb trailer for half a tank at highway speeds.
Best I ever did was cruise control for the whole tank, average 70 mph, 31.3 mpg with 2 people and a full trunk of luggage.
Average over the life of the car (I track it in a big spreadsheet) is 20.6 mpg on 91 octane gas.
All mpg measurements are adjusted for your U.S. gallons.
The EPA fuel economy ratings, for our cars, were 24 city/35 hwy (Imperial gallons) or 20 city/29 highway (U.S. gallons).
My average fuel cost over the life of the car, at the prices I have paid for gas, is 8.1 cents (Cdn $) per km, or approximately 8.2 cents per mile (US $).
1.0 gal US = 0.83 gal Imperial
Kerry