Plug-in electric vehicles are poised to become practical and important, which is why we’re seeing a crescendo of reports about their “MPG” fuel economy. Unfortunately, these reports raise more questions than they answer, including two questions that will occur to informed consumers: “How is this ‘MPG’ value defined?” and “What does it mean for me, given how I drive?”.
In evaluating and comparing vehicles that are all powered only by gasoline, miles-per-gallon (MPG) makes great sense. But MPG isn’t what it used to be. As discussed in Eric Cahill’s recent post (here), MPG is obsolete for two main reasons: the growing popularity of alternative fuels, and the emergence of vehicles powered by multiple fuels (most importantly electricity plus gasoline or other liquid fuels).
Comparing the MPG of gasoline with the MPG of ethanol (to use one example) is like the proverbial comparison of apples to oranges; besides, what’s a gallon of electricity? For a Plug-in-Hybrid-Electric-Vehicle (PHEV), MPG is even more misleading (at best, it tells just part of the story – e.g., accounting for just the liquid fuel but not the electricity).
As the figure of merit for fuel economy, the Progressive Insurance Automotive X PRIZE is using MPGe (miles per gallon energy equivalent), defined as:
MPGe = (miles driven) / [(total energy of all fuels consumed)/(energy of one gallon of gasoline)])
MPGe is a simple, well-defined measure of overall vehicle efficiency. MPGe is easy to explain, accounts in a neutral manner for any combination of fuels, and reduces to the familiar MPG in the case of gasoline fuel only.
To make it easier to understand (and get some intuition about) MPGe, we’ve developed a spreadsheet available here. It includes three simple calculators:
(1) Enter the distance driven and the amounts of all fuels consumed – result is MPGe. The formula for this calculation is the one given above.
(2) For the special case of gasoline-electric PHEVs, enter the gasoline usage (MPG gasoline) and electricity usage (watt-hours per mile) – result is MPGe. The formula for this is given and discussed at length in my blog post available here.
(3) For the case of any plug-in vehicle, enter the charging time and the charging efficiency – result, for three different electric circuit types, is the total energy added to the battery, expressed both as kWh and as (energy-equivalent) gallons of gasoline.
This is a hard time for the automotive industry. But it’s also an exciting time of change and innovation. Consumers will soon have a variety of energy efficient and environmentally friendly vehicle choices as diverse technologies and alternative fuels come to market. This diversity will make it harder to evaluate and compare vehicles, and how best to do so is a difficult question. MPGe is not the entire answer, as it must be presented in the context of driving style, frequency, and distance. But we believe it’s part of the answer. MPGe should be the new MPG.