I use a site that monitors fuel prices and has a fuel log book that you can use for your vehicle. The data is returned in L/100km or MPG (Imperial) Heres the URL:
Total Quantity Purchased (Litres) Total Cost ($) Average Price (c/L) Average Fuel Economy (MPG) Total # Fill-ups 1462.352 $1,339.90 0.923 30.91 43
or
Total Quantity Purchased (Litres) Total Cost ($) Average Price (c/L) Average Fuel Economy (L/100km) Total # Fill-ups 1462.352 $1,339.90 92.3 9.099 43
Best Tank: 37.44MPG (7.5L/100km) Worst: 20.08MPG (14L/100km)