Forget MPG and MPG-e

May 30, 2018

How turning a metric on its head may help us think about performance.

Recently I have been thinking about how our point of view for electric vehicles is almost entirely embedded inside of our one hundred yearlong familiarities with the internal combustion driven vehicle.

We talk about performance in terms of “miles per gallon or in Europe in terms of liters per 100 kilometers. As electric vehicles have begun gaining traction over the last decade it was only natural that we would start referring to their fuel efficiency in miles per gallon equivalent.

This of course is a total misnomer as EV’s do not use any liquid fuels for motive power at all. Since there are 111,836 BTU’s in a gallon of gasoline (E-85) and there are 3,412 BTU’s in a kilowatt hour (kWh) we simply divide 111,836/3,412 equals 32.777 kWh in a gallon of gasoline equivalent.

Needless to say this can be very confusing to most people. It really is something equivalent to telling folks the outdoor temperature in Celsius instead of Fahrenheit in the United State. Even if you know the conversion factor you generally cannot do the math in your head.

The real problem stems from having to do a conversion at all. If you only live with one measurement system all your life you have a mental picture of what the number means. If someone says it is 37 C out and you live in Europe you know it is damn hot out. That is 100 F to you and me in the states.

Someday when electric cars are more dominant in the market place and fossil cars are being abandoned we will have to start measuring energy performance for EV’s in a more relevant format. For us in the United States (assuming we are still using the Avoirdupois measurement system) this would be in kWh per 100 miles. In Europe this would be in kWh per 100 kilometers.

Since the whole metric problem is just matter of what you are the most familiar with I can imagine that someday people will develop a puzzled look on their faces when you tell them about the energy performance of the fossil car in terms of miles per gallon. They just won’t be able to picture it or do the conversion.

For that reason I decided to make up a small table giving the energy performance of a series of different class of ICE vehicles and comparable BEV’s in kWh per 100 miles. You can still compare them directly even though the numbers may look a bit strange.

Combined/City/Highway kWh per 100 miles ICE

Honda Fit Chevy Spark Nissan Versa Toyota Camry

93/137/78 99/143/75 102/143/81 102/164/67

Honda Accord Hyundai Accent Toyota Corolla Honda Civic

106/156/78 106/164/81 102/143/76 102/156/72

Toyota Avalon RAV-4 Hybrid Honda CRV Toy. Highlander

137/205/96 106/126/91 117/164/89 149/219/102

Hon. Ridgeline Ford XLT 2.7 L Chev. Silverado

164/252/113 173/252/126 205/298/143

Combined/City/Highway kWh per 100 miles BEV

Tesla-S P85D Tesla-X 90D Chevy Bolt Nissan Leaf

38/57/33 36/41/39 31/29/36 33/29/36

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