Confusion Over MPG Ratings for Electric Cars

“What will fuel-economy numbers look like on the window stickers of electric vehicles coming to the US market next year?” asks Paul Weissler, in his recent article for the Society of Automotive Engineers. Trying to find the answer could short-circuit your brain.

The big numbers on the window sticker for BMW’s Mini E—currently in the hands of hundreds of consumers in a test program—are 33 in the city and 36 on the highway. But those are posted in kilowatt hours per 100 miles. The smaller text explains that the equivalent is 102 mpg city and 94 mpg on the highway.

When plug-in cars hit the US market in the next year or two, consumers will need a lot of help deciphering the efficiency figures of vehicles that carry electric fuel by the kilowatt hour rather than liquid fuel by the gallon. Nissan’s upcoming yet-to-be-named electric car, according to some tests, will get 367 miles per gallon. The Tesla Roadster is reported to get 135 miles per gallon. And the Chevy Volt plug-in hybrid…that depends.

If the EPA uses tests designed for electric cars to evaluate the Chevy Volt, the ratings could exceed 100 mpg. But if the government agency classifies the Volt as a hybrid and tests it as such, the EPA rating would drop to about 50 mpg. The difference could mean success or failure in the marketplace. Corporate Average Fuel Efficiency and sticker numbers for plug-in hybrids, which use gasoline and electricity in various degrees and ways depending on the specific vehicles design, have not yet been determined.

Plug-in Car Driver Recharging His Car

It will take time for consumers to learn a new system for evaluating the efficiency of cars that use little or no gasoline. That system has not yet been fully developed.

Today’s conventional hybrid gas-electric cars faced similar hurdles soon after they became popular. Some owners were disgruntled when they didn’t achieve the level of the Environmental Protection Agency numbers—such as the second-generation Toyota Prius’s initial EPA city rating of 60 miles per gallon. In 2006, the city and highway numbers were adjusted down to the 40s to more accurately reflect real-world driving. But the shift to powering cars from the grid will bring unprecedented levels of uncertainty and confusion.

Weissler explains that convention vehicles including Prius-style hybrids are derived from EPA dynamometer emissions lab tests—and then adjusted according to various legal mandates. For all-electric cars, the CAFE calculation begins with converting gasoline’s energy content into electrical terms—based on “multiplying factors” for the efficiency of electricity. The Department of Energy slightly increases the resulting CAFE number because it doesn’t directly use petroleum, but if the car uses any “petroleum-fired accessory, such as a heater,” the number is brought back down. All tests apply a 55/45 weighting on numbers derived from city test cycles rather than highway driving.

There are also inconsistencies between methods and calculations used by the EPA and the California Air Resources Board. The Mini E’s equivalent numbers are based on the CARB’s “petroleum energy content-equivalent” of 32,600 watt hours per gallon, rather than the EPA’s petroleum equivalent of 33,705 watt hours per gallon.

The Mini E’s EPA city and highway numbers averaged out to 98 mpg—but the fate of Nissan’s upcoming electric car is still uncertain. Weissler writes: “If the Nissan-measured weighted average is 223.57 W·h/mi, dividing 82,049 W·h/gal by 223.57 W·h/mi would equal 367 mpg.” Easy, right?

Finding a way to provide easy-to-grasp efficiency numbers to the non-technical general population of car buyers will be difficult. But it’s one more essential task necessary for adoption of cleaner greener electric-drive vehicles.


  • dan2931

    why not range per charge? city vs Hwy?

  • crookmatt

    Dan,

    I think that miles per charge is an important data point when purchasing an electric vehicle, however it is not directly related to efficiency, only indirectly related in conjunction with battery capacity. In other words a less efficient electric vehicles could still have more miles per charge (i.e. electric range), than a more efficient vehicle just by having a larger battery.

    We see this in gas powered vehicles today. While the Prius is the most efficient gas burning car, there are several models that get more miles per tank, simply because they have larger tanks.

    I don’t think it’s practical to compare efficiency numbers based on emissions since emissions for an electric vehicle depend heavily on the source of electricity.

    Unfortunately comparing cost of refueling per distance traveled would be difficult since gas and electric prices are constantly changing.

    Something like the proposed Kwhr/100 miles probably makes the most sense, since it will allow to accurately compare one electric vehicle to another electric vehicle.

  • FamilyGuy

    I like miles/$0.01. The numbers in big print would need to have the * to show what the assumed value of the charge would be. The perhaps a small grid saying…if gas cost $x.xx per gallon and if you recharged your car at the following rates x. Then maybe the consumer would have an idea. The end of the day, can I assume that people want be off gas (foreign oil)? and how much does it cost me to drive a miles? How many miles can I get from a penny, a buck, or $10 unit of payment?

    For some hybrids, assume full change on the battery and full tank of gas. Then drive the car until it drives no more. How many miles did it drive? How much did it cost to recharge it? How much did it cost to refill the tank? That would give you a cost per x miles driven. This could be consistant for gas, gas-electric, electric-gas, and all electric (and even fuel cell)? Just a thought.

    But some in the EPA has to have thought of this……

  • Mr.Bear

    They can assume an electricty cost the same way that fridges are rated in both kW/mile and estimated $/mile. But there will need to be an mpg conversion so someone can compare an EV to a hybrid or gas vehicle.

    This is OT, but one thing I don’t hear anyone talking about is how much the range of an EV will be cut down by cold temperatures. I have an unheated, detached garage. In the winter it can drop below 0F. I’m pretty sure I wouldn’t be able to charge the car as much as on a 70F day.

  • Mr.Bear

    Sorry, editing mistake. Should be “cars can be rated in kW/mile and $/mile the same way fridges are rated in both kW/year and $/year”.

  • Chip Daigle

    I think they need a similiar way of accounting for E85 cars using only 15% gasoline for its MPG. In other words your mpg of E85 blended gasoline should be multiplied by 6.67 to get your true MPG (of gasoline.) We are making our country cleaner by using Ethanol and really cleaner if the Cellulosic Ethanol makers ever get off their butts and start producing. We also make our country stronger by using Ethanol because we dont have to buy as much Oil from the Mid East and we can flip the switch when the price of gasoline spikes because of some fake crisis in the Mid East.

  • sean t

    Currently, we use MPG in the US and l/100km elsewhere.
    There are some discussions re MPG vs l/100km but it’s another matter.
    The MPG has dimension of distance / fuel and l/100km has fuel / distance.
    If we’re to use the above dimensions, it’s very hard because it depends on the source of electricity. How can we measure the fuel for solar energy? Hours of sunshine? Hence, we can settle with distance / energy or energy / distance which can be Mile Per HorsePowerHour (or 10 HPH or 100 HPH) or KwH / 100 km (or 10 or 1 km).
    Just a thought.

  • Dj

    FamilyGuy has a very good idea…..about how to rate gas and electric cars…..I say again a very good idea!!!!!!!

    Dj

  • ex-EV1 driver

    As a current and past EV driver, I, personally like miles/Kwhr or whr/mile. miles/Kwhr is similar to Miles/gallon which Americans are already used to.
    In the case of my current ride, the Tesla Roadster, it displays between 250 and 330 whr/mile or 4 to 3 mile/Kwhr (depending on driving style).
    $/mile is a bit confusing or misleading because of the wide variation on price per Kwhr of electricity. In some places it is as low as 8 or 9 cents per Kwhr while in others, like Southern California, it can be as high as 35 cents per Kwhr that varies with consumption amount or time of use, depending on where you live and what kind of plan you have.
    Of course, if one has rooftop solar cells, one is interested in miles/Kwhr as you’ll know how many Kwhr your solar array puts out and you can simply multiply it by the m/Kwhr to find out how many solar miles you can drive per day.

  • sean t

    ex-EV1 Driver,
    Yeah, dollar term is not good because it’s affected by many factors.
    And in different countries, you’ve to use different units of money . . .

  • Aaron F

    A mini E at 33/36 kW-hr/100miles vs. a new Prius at 50mpg

    Let’s say electricity cost is .10/kW-hr then the mini-E will cost $3.30/3.60 per 100 miles driven (more if you consider recharging loss)

    Let’s say that gasoline cost $2.50/gallon then the Prius would cost $5.00 per 100 miles driven.

    If the Mini-E was upsized to the size of a Prius, I’m guessing the super-sized mini-E would cost closer to the Prius per 100 miles driven. Add in limited driving range and recharge time and cost and I’m not sure I see the big benefit of an electric car over a hybrid car, at this point in time.

  • Mr. Fusion

    “Confusion Over MPG Ratings for Electric Cars”
    The title of the article shows how confused the world is. There are no gallons to be replenished in an EV.

    It’s easy. MPC (Miles Per Charge) That brings the numbers back to an understandable order. Big numbers mean more, little numbers mean less.

    For plug-in hybrid vehicles have two ratings. MPG and MPC with a big fat total on the bottom.

    As far as how much it costs to charge, figure it out on your electric bill. We buy gasoline cars today with no cost of gas on the sticker, we just figure it out with our common sense and wallets.

  • Dan L

    kilowatt hours per 100 miles should be the official measure. It describes the vehicle. It does not describe anyone’s assumptions about how much gas costs, or electricity costs, or how much gas it takes to generate electricity. It will be a useful measure long after all of these assumptions change.

    In order to help buyers understand what they’re getting, the sticker can also contain conversions to mpg, mp$, $/year, etc., with the assumptions used in these conversions explicitly stated.

    I like the format of the sticker shown above.

    The contents are another matter. 33kwh/100m is pretty disappointing.

    Hmmm… I wonder how to abbeviate kilowatt hours per 100 miles. Not kwhphm …

  • Andy

    I agree with Dan. Perhaps WhPM (Watt Hours per mile) would work. Range is no good since it is essentially the size of battery or gas tank though weight and efficiency have a part there. This would be an apples to oranges issue.

    I do think that for PEVs that have extended range ICEs they should be rated with a MPG during their full time operation. Thus a sticker might read:

    330 WhPM/50 Mpg

    It will be interesting to see how this plays out.

  • Pickey McPickey

    Marketing Rule #1
    We need to speak to people as if they had a 5th grade education:
    Most consumers want to know only 3 things:

    How long can I drive it before I have to re-charge?

    How long will it take to Fully Recharge? (110 & 220)

    How much (on average) will it cost me to recharge based on average electricity costs and driving miles of 12,000 miles per year?

  • sean t

    Pickey,
    They may be a 5th grader but not totally un-educated.
    If the current unit is MPG or l/100km elsewhere then what is wrong with KwH/100Km or KwH/Km? Or Mile per KwH?

  • Libor

    Pickey,
    1) The “5th graders” will be able to compare apples with apples. Trust them. The more energy it uses, the worse it is. 60 is more than 30, 30 is more efficient.
    2) You really need a solid, un-debatable scale for efficiency – i.e. government agencies need rock solid data for their decisions, journalists need it for good reviews, and I belive that most of the people want to have true data.
    Don’t forget, that even in case of most complicated scale you can ever imagine, there will be plenty of marketing rubbish written on leafelts, advertisements, etc. Reviews will be written, experience shared.
    So a big YES for Kwhr/100km or similar unit.

  • Jon

    Wow… you’ve been drinking that ethanol cool-aid haven’t you!

    Perhaps they could factor in all the petro based fertilizers that go into Ehtanol too ;) oh and the federal tax dollar subsidies…. and they could reduce the efficiency for Ethonal’s lower power content…

    Seriously, do you really need even more complicated labels to help your buying decisions? Couldn’t one just do the consumer research themselves?

  • Priusmaniac

    It looks rather simple for pure EV vehicles; miles per KWh is quiet convenient because it gives a value like 5 or 6. Otherwise, for a plug-in hybrid, I would use miles per KWh in EV mode and mpg for the hybrid mode. Although in the hybrid mode, there is also interest in the KWh/gallon value, which measures the efficiency of the onboard electricity generator. This can start at 10 KWh/Gallon and go up to 20 KWh/Gallon or more.

    A completely different way of looking at it is fossil CO2/mile. Which can be zero for both EV mode and pure non-fossil fuel hybrid mode. For example using E100 or B100. Although the entire chain without fossil CO2 is difficult. In the case of the EV mode, producing the steel for the pylons transporting the electricity and the copper for the wires still represent some fossil CO2. The same can be said for the E100 because of fertilizers and transport, although bio agriculture combined with E100 powered process and transport can be an answer to that.

  • EVO

    kilowatt hours per 100 miles IS, IN FACT, already the EPA/DOE official measure for electric drive vehicles.

    An example is the official EPA efficiency sticker on a new mini-E.

    MPG is a meaningless metric for anything with 100% electric drive. It’s at vehicle energy use efficiency that the EPA tries to report, and kWh / 100 miles gives it to you for electric drive.

    The smaller the number of kWh / 100 miles the vehicle uses, the more efficient the vehicle is. Simple.

  • Perspective

    Since, at vehicle level energy use, an internal combustion engine (ICE) runs at around 25% efficiency, real world, and an electric motor runs close to 100% efficiency, most folks will be very hard pressed to believe that a vehicle that uses both, such as the Volt, could in any way be rated as more efficient (230 mpge) than a vehicle that uses only a more efficient electric motor, such as the Mini-e (33 kWh / 100 miles ~ 102 mpge).

    Someone has some ‘splainin to do. It’s just not credible that anything with an ICE as part of its energy use could be more efficient than an all electric, at the vehicle level, which is the level EPA has always reported.

    hint: If you are going to add in original energy source, electric transmission, distribution or market costs for electricity for electric vehicles, you also have to add in all military, transportation, distribution and other crude oil costs since the start of the last century for vehicles that use gas to be fair.

  • Macz

    I’m with redbeard and picky,
    Keep this simple. All anybody really wants to know is how far can I get before the car needs re-fueling, regardless of the fuel.
    Why so complicated?

  • David Raikow
  • Richard1

    I was wondering if the EPA looked at the BTU’s required to generate the KWH that are used to charge the batteries in the electric vehicles?

    When you take into account the power plant efficiency, transmission line losses, the efficiency of the battery chargers the numbers don’t look as good.

    The Volt on its batteries run in the 15 to 20 mpg range and the Leaf run in the 25 to 30 range whwn looking at the BTUs needed to generate the KWH.