Many have suggested that we use cost-per-mile rather than MPGe as the primary figure of merit for judging Progressive Insurance Automotive X PRIZE vehicles. That’s appealing, since out-of-pocket costs are a primary concern of most consumers (although we hope they’re also concerned about the costs to society of fuel-related energy consumption and emissions). But while cost-per-mile is appealing in principal, in practice there are disadvantages, including:
- The traditional (gasoline only) MPG is well understood by the public (and generalizing the concept via MPGe is a straightforward proposition), but studies have shown that translating MPG into cents/mile takes place in less than 5% of households.
- 100 MPGe is a meaningful, round number. It is hard to imagine too many headlines about 3 cents per mile.
- Cost-per-mile does not just depend on a vehicle’s efficiency – it depends very much on when you buy fuel and where you buy fuel. That makes cost-per-mile figures problematic for consumers, and problematic as a means for comparing vehicle technology.
We chose MPGe because it makes sense as a generalization of the familiar MPG, and because we wanted an unambiguous, soundly-based, figure-of-merit that depends only on things the vehicle designer can control. We then supplemented MPGe with our second figure-of-merit, “wells-to-wheels” greenhouse gas (GHG) emissions, which addresses environmental and other concerns.
That said, during the competition we will publish numerous figures of merit in order educate consumers about many aspects of the competition vehicles, likely including:
- MPGe (also used for scoring)
- GHG emissions (also used for scoring)
- GPMe - gallons-per-mile energy equivalent (this is 1/MPGe)
- MPG - miles-per-gallon of liquid fuel, typically gasoline
- GPM - gallons-per-mile of liquid fuel, typically gasoline (this is 1/MPG)
- Amount of petroleum used (wells-to-wheels - i.e., including production and distribution)
- Amounts of all fuels used
- Fuel cost per mile (based on current national average prices)