YES I totally forgot that one, I've thought about it a few times... really all you need is a flow meter for the fuel line and MPG can be calculated from that and the odometer. Average of course, instant you'd have to get a feel for what the gauge is saying.
I don't always feel like emptying my gas tank and then filling it up completely just to get an accurate average MPG calculation, it's also impossible for me to drive an entire tank without getting into the secondaries I can't help myself. That would just be a waste lol, after all my Duster is my "fun" car.
