Man some of you are making it so complicated.
Fill it up drive, divide miles driven by amount of gas used = MPG .Average a couple tanks if you want exact MPG for a time period.
Not rocket science 
Last tank I got 17 MPG mixed driving only 60-70 MPH on highway with running it 2 times for 5 minutes 4 times a week for dvd so not to stress battery(wife uses dvd during lunch time).No auto start used during last fill up.
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