I think boethos wants some way to correlate his efficiency with his car's mileage. I could imagine another label along the bottom of the Recent Fuelups graph that displays the car's mileage for each fuelup.
People who currently don't track with the odometer would need to set some start date to their mileage, such as "Today my odometer said X", and then the system would fill in their mileage for past fuelups. Then they could keep using the tripometer method.
boethos 
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