You can log your fuel purchases and mileage at www.gasbuddy.com and it does the calculations for you. You can also keep track of gas prices in your area.

I've been using it for a few years now and I keep track of all my vehicles here.

My 2008 X5 3.0 averages between 16 and 18 mpg per tank.

__________________
2008 X5 3.0 - Sapphire Black Metallic, Tobacco leather, Sport, Premium, Cold Weather
2002 E39 M5 - Carbon Black Metallic, Black leather
2013 S1000RR - Insanely fast!!