The right way to interpret MPG

19Jan09

The fuel efficiency of a motor vehicle is popularly measured by miles per gallon (MPG). The ubiquity of this measure is such that the price people are willing to pay for a car is linearly correlated with the car’s MPG value. But, if you want to save the environment or reduce spending on fuel, the better measure is GPM — gallons used per miles travelled. The price you are willing to pay for a car should therefore be linearly correlated with GPM, and hence not linearly correlated with MPG, its reciprocal. In this short paper, the authors conduct a few experiments to establish the linear correlation of perceived environmental unfriendliness of the car with its MPG value, and suggest that we really should not be too leery of incentives like giving tax breaks to SUVs that use hybrid engines — for the absolute amount of fuel saved by changing your vehicle’s MPG value from 14 MPG to 12 MPG is actually, contra the linear interpretation, more than that saved by you switching from a 28 MPG vehicle to a 40 MPG vehicle. A small increase in MPG for a fuel-inefficient vehicle is, in environmental terms, as beneficial as a large increase in MPG for a fuel-efficient vehicle.

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