How do you measure the mpg on an electric motor?

Question

Recently there have been announcements by GM's Chevy dept that It's volt receives some 230 mpg and the Nissan also announced that it's Leaf gets 367 mpg. My question is how does one calculate the mpg rating on an electric car... I thought I'd search online to find a conversion from mile per charge to mpg, but haven't found anything...anyone got an answer?

Answer 1

GM and then Nissan used the EPA's draft method for calculating an mpg rating from an electric car.



It's a difficult situation, because people are used to seeing a car's efficiency in mpg, but obviously that's not applicable to a car when it's not using gasoline. So what the EPA is doing is figuring out a conversion to translate the amount of energy used by electric cars into an equivalent mpg rating. But they haven't finalized the conversion yet. All they have at this point is a draft, and GM took the draft method, calculated a value for the Volt, and made a big deal about it. Which is kind of lame. Then Nissan one-upped them by using the same calculation for their LEAF.

Answer 2

First mpg ratings probably do not mean too much to your driving. It is like looking at the odometer of a used car. There can be tricks involved:



I don't know anyone who in a real life situation gets the mileage that their vehicle is "rated" at. At best it is a comparison figure between vehicles. But comparisons and certain figures of themselves can be misleading. When advertisers use these figures, they can be intentionally misleading.



One of the initial ways that electric cars were rated was kWh used for 100 miles. Why? Why not 200 miles or 50 miles? It turns out that this produced a number that looked a lot like a reasonably good number for mpg. A quick reading by someone not looking carefully could easily confuse the two. Here is a site that shows an original RAV4 ev sticker showing city consumption at 27 and in smaller print kw-hr/100 miles. The highway consumption is listed at 34. (1) I am old enough to remember when pump signs were briefly switched to liters and the price seemed to drop dramatically.



Then again I have seen a lot of mileage claims for hybrids that only list the liquid fuel consumption. So if you plug in your car and then only use 1 gallon in a month as a result all the mileage for that month is attributed to that one gallon when in fact it was running off electricity that you were also paying for, just not at the pump.



For an electric vehicle it is not difficult at all to convert from one to the other (sort of.) You remember having to find he common denominator when adding fractions. Or maybe later having to convert problems to the same units of measure. The same concept applies here. You need a common measure between electricity and gasoline. BTU is used. The British Thermal Unit is a measure of heat content. The calorie is a similar measure using the metric standard.



It turns out that we can measure the BTU in a gallon of gasoline and find it is about 114,000 BTU and 1 kWh of electricity equals 3413 BTU. Other fuels or energy sources can also be measured and there is a resulting GGE (Gasoline Gallon Equivalent with 33.4 kWh = 1gal of gasoline [3413/114000]) 2 & 3



The numbers that the EPA gets are theoretical. No one is getting in a several hundred cars and averaging the results. This means that assumptions are made about the driving conditions. An LA. highway that has turned into a parking lot with traffic congestion is a different kind of city driving than rush hour in Enid, Oklahoma. Assumptions are also made about general driving techniques. You have no doubt heard of people who have a knack of getting 100's of miles out of a gallon of gas by using tricks like coasting, never touching the brakes... And assumptions are made about the drag of the wind and the drive train. A good tailwind, "dragging" behind a big vehicle, and regular maintenance may alter this dynamic as well.



Any vehicle with regenerative braking and a means to store and reuse the resulting inertia energy captured is probably going to have better mileage in stop and go low speed city driving than high speed highway driving.



See my other answer cited below for 3 conversion examples of electric cars. Many electric vehicles will say that they can get a certain mileage on a full charge. Owners find that their experience is never quite so good. This has a direct effect on the mileage" in the same way as traveling 150 miles on a tank of gas is a whole lot different than traveling 300 miles on the same tankful. The Tesla for example claimed a mileage of around 225 miles on a full charge and owners find that it may be closer to 160 again depending upon the driver and the conditions.



And since you are comparing electricity to gasoline you should also take into consideration the different cost of each. Gasoline is currently less expensive for the amount of energy it holds but electric vehicles will always be far more efficient than gasoline powered vehicles leading to cheaper operating costs with electric vehicles.

Answer 3

EPA has not released their methodology but it probably has something to do with the energy released by a gallon of gas converted to electricity and how far a gar can go on that energy.



So the LEAF goes about 367 miles on the energy of a gallon of gas.

Answer 4

At the moment it is greenwash, there is no relevant conversion, especially for series hybrid like the Volt that changes comsumption depending on how far your journey is.



A better measure would be grams CO2/mile or km; http://evworld.com/article.cfm?storyid=1745 and it doesn't take into account other pollutants like particulates, noise, off-gassing, petrol stations & refineries etc.

the media & motor industry are not prepared to educate people on what this means, as to do so would be to acknowledge that infernal combustion is incredibly inefficient motive power, very polluting, high maintenance, dependant on vulnerable finite supply routes etc.

Answer 5

the two AC and DC autos could be variable velocity. Variable velocity and variable torque. in case you utilize an AC motor, your gazing approximately 10 to twelve thousand funds for the motor, 3 thousand for a controller or it, an a large adequate generator could run yet another 10 thousand. and as the burden of all this you will desire a enable like a 18 wheeler to apply the roads, some greater thousand. you ought to get via with around a hundred thousand

Answer 6

The mpg on the electric motor is non-existent since it uses no gas. The mpg is for the whole powerplant which includes a 1.4L gas engine, which is used to charge the batteries.

Answer 7

http://www.hybridcars.com/fuel-economy/chevy-volt-230-mpg-rating-what-does-it-mean-25986.html

has some details, including a link to more.



Apparently *other* electric cars can claim even higher numbers, using the same methods.

Answer 8

From what I hear the EPA has not yet released their methodology for calculating the mpg of plug in hybrids.



Something relevant:

http://www.cleveland.com/business/index.ssf/2009/08/post_35.html

Answer 9

the same way as a normal car.

they stick a device in the tail pipe that reads the co2 emissions so hybrids using there batteries automatically get a leg up when it comes to mpg but you wont really get that many miles to the gallon they dont measure like any normal person would by filling it up and seeing how long it runs, they measure the amount of wasted fuel that comes out as co2 so those cars dont run like their companies or the epa says they do.