Goodbye Miles per Gallon, Hello Monthly Fuel Cost

We are all used to dealing with miles per gallon (mpg), that familiar if sometimes clumsy indicator of a vehicle's fuel consumption. The Environmental Protection Agency (EPA) provides mpg ratings, along with the disclaimer that our actual mileage will vary. And it always does.

That's just one indication of how flawed mpg ratings are. But don't worry. They will soon be rendered completely obsolete by advancing technology. We think it's time to get on with the changeover to something better.

A figure that reflects monthly fuel cost makes much more sense. Consider the coming plug-in hybrids, which run a significant distance on electricity alone, or pure electrics, which run entirely on energy drawn from a wall socket. There are no gallons of anything to measure. Compressed Natural Gas (CNG) is dispensed in a gallon-equivalent fashion, but what you're really buying is cubic feet of a gas. And what about alternative fuels such as E85 (a mix of ethanol and gasoline)? Or even diesel, for that matter? They are delivered by the gallon, but they involve such different costs and energy densities that comparing mpg figures of cars powered by such fuels to those of a gasoline vehicle is at best incomplete and at worst confusing and misleading.

Trying to force everything into gasoline-equivalent miles per gallon (MPGe), as the EPA seems to favor, is to stick one more Band-Aid on a system that was pretty lame to begin with.

Engineers can come up with equivalency formulas to allow meaningful direct comparison of all these different energy sources. But this comparison should not be made on an energy-content basis. It should compare what consumers care about most, which is cost.

Edmunds believes vehicle energy consumption should be expressed as Monthly Fuel Cost (MFC). What will it cost to fuel up — or more accurately "energy up" — a vehicle in dollars per month, no matter how that energy is stored and delivered? That's what we really want to know. A common MFC figure will make all fuels and energy sources easily comparable, and most directly support consumers' efforts to manage their transportation budgets.

Any standardized formula requires certain assumptions, and that means, in essence, that the thing we used to call "your mileage" will still vary. But using typical miles driven in a month and national average prices for electricity and all the various fuels, we can calculate an MFC figure that allows meaningful comparison between vehicles and technologies.

To begin the transition from mpg to MFC, we offer the tables below. We have calculated MFC for a wide range of vehicles. We call out in a separate box some alternative-technology vehicles to highlight how they compare in terms of MFC. For purposes of calculation, we assume 1,250 miles driven per month, and use regularly updated national-average electricity and fuel costs as provided by AAA or the Department of Energy.

We think you will quickly realize how useful this method of comparison is. Hopefully the EPA will recognize the same!

Edmunds' CEO Jeremy Anwyl sent a letter to the EPA, appealing for consideration of MFC to replace MPG. Read the full letter here.

Monthly Fuel Cost Examples

Vehicle Vehicle Technology EPA City & Hwy
Combined MPG
Edmunds Monthly
FuelCost (MFC)
2011 Toyota Prius parallel hybrid 50 $90
2011 Honda Civic GX CNG - Natural Gas 28 $55
2011 Ford Fusion Hybrid parallel hybrid 39 $115
2011 Jetta TDI diesel 34 $140
2011 Ford Focus gasoline 28 $158
2011 Honda Accord I-4 gasoline 25 $172
2011 Chevrolet Impala 3.5L on gasoline 26 $193
on E85 19 $233
2011 Chevrolet Silverado 5.3 V8 Flex Fuel on gasoline 17 $261
on E85 13 $307
2011 Chevrolet Volt** series plug-in hybrid 93 (Electric*)
37 (Gas)
2011 Nissan Leaf electric 99* $41

Note: energy costs from U.S. Dept. of Energy and AAA
* MPG Equivalent calculated based on energy content

** ** Volt fuel cost estimate assumes battery is fully drained and re-charged daily.

Monthly Fuel Cost Data

Download a Microsoft Excel file containing Monthly Fuel Cost for all vehicles, from Edmunds AutoObserver. Read more

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