Monthly Fuel Cost vs. MPG: Edmunds.com's Letter to the EPA

It's Time for MFC to Replace MPG


There is a debate brewing about the familiar EPA mileage ratings.

Since 1975 the EPA has calculated both city and highway ratings that appear on the window sticker of every new vehicle in America. As the EPA pointed out with the phrase "Your mileage may vary" the ratings had limited value as a predictive tool. But they undeniably have been very useful for consumers looking for an "apples to apples" way to compare vehicles when looking to make an informed choice.

Since launching, the methodology has been tweaked to try to improve the connection between ratings and real world experience, but the underlying idea has remained consistent.

But time and technology marches on. For an increasing number of vehicles, "gallons" aren't being consumed at all. What is the MPG of an all-electric vehicle, for instance?

Proposed is an array of energy equivalents backed by complex algorithms — all designed to translate energy consumed into the now familiar MPG rating. These will no doubt be elegant and greatly impress engineers, chemists and physicists. But I fear they will serve to confuse consumers.

So here's my suggestion: Start over.

Trying to convert various forms of energy to a standard is missing the point. Stepping back from the issue, it should be clear that consumers have used the existing MPG ratings primarily to get a sense of the relative cost of operating a vehicle on a day-to-day basis. MPG is not a direct measure of costs, but as long as fuel quantities are measured in gallons, MPG provides a relative guide. A vehicle that consumed more gallons costs more to operate. Simple enough.

With energy equivalents, the picture is anything but simple. Some of the draft data I have seen can easily cause consumers to draw erroneous conclusions. For example, a new Toyota Prius has a combined 50 MPG. GM is reporting that its new Chevy Volt will possibly be rated at 230 MPG. From this we could reasonably assume the Prius is more than four times more costly to operate than the Volt. But we would be wrong.

Let's look at a selection of new and old technologies and how they might be rated with MPG or a MPG equivalent versus our proposed fuel cost per month rating.

Make and Model Vehicle Technology EPA Rating Edmunds.com Monthly Fuel Cost
2009 Mini E electric 99 mpge $49.39
2010 Toyota Prius parallel hybrid 50 mpg $67.78
2009 Toyota Prius parallel hybrid 46 mpg $72.58
2009 Honda Civic GX CNG 28 mpge $77.23
2010 Ford Fusion Hybrid parallel hybrid 39 mpg $85.61
2010 Jetta TDI diesel 34 mpg $103.38
2010 Ford Focus gasoline 28 mpg $119.24
2010 Honda Accord I-4 gasoline 25 mpg $133.55
2009 Chevy HHR Flex Fuel on gasoline 25 mpg $133.55
on E85 18 mpg $152.22
2010 Chevrolet Silverado 5.3 V8 Flex Fuel on gasoline 17 mpg $196.40
on E85 13 mpg $210.77
Chevy Volt @ 40 mpg gas series plug-in hybrid 230 mpge $53.55
Volt estimates: 40 mpg combined on gasoline, 40 mile range on 9kwH electricity, 15% charging losses, 0.62 utility factor

View monthly fuel cost for all new vehicles.

Looking at this analysis we find that electric vehicles do enjoy a cost advantage over their counterparts powered with other technologies — but this advantage is nowhere near as great as the proposed EPA ratings would imply.

It should be noted that the EPA already provides the "annual usage costs" data shown on the current window stickers. So in a way we are proposing a shift in emphasis. Instead of featuring MPG or MPGe, feature monthly costs. But the key here is that when using EPA figures in marketing, automakers would be required to use EPA provided monthly cost estimates.

To make this work, the EPA will need to define a driving circuit that reflects the activity of a "typical month" — including commuting, outings and errands. Where needed, standards will also have to be defined for charging efficiency and other energy-specific assumptions.

A potential criticism is that ratings will require assumptions about the future costs of various sources of energy — assumptions that would change annually. This adds some complexity, but it is also aligned with reality. These costs do change and consumer decision-making needs to be assisted with data that reflects this reality. Besides, with the Internet it would be a simple matter to build a calculator that allows consumers to get up-to-the-minute data based on their own mileage estimates.

A final note: There are other sets of energy/environmental usage data points that consumers are increasingly concerned with: carbon usage, recyclability, use of renewable energy, etc. These are important measures and we encourage the EPA to develop and publish additional corresponding measures to aid consumers in their comparison shopping.

But, please, keep them simple.

Sincerely,

Jeremy Anwyl
Chief Executive Officer
Edmunds.com

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