Cooley On Cars
Car Tech 101: MPGe explainedBrian Cooley drills down on the new metric for rating MPG in cars like the BMW i8, Nissan Leaf and Chevy Volt.
[MUSIC] Now, of course, we all recognize the MPG in MPGe, but that's the easy part. But that little lower case e at the end, that's actually surprisingly powerful. Let's break down the acronym, first of all. MPGe stands for miles per gallon of gasoline. Equivalent. Now the equivalent part is where we're talking about energy in a gallon of gas as opposed to the volume of a gallon of gas, which is what MPG basically refers to. Here's how they work that out numerically. The EPA says that a gallon of gas has 115,000 BTUs, or British Thermal Units, of potential energy in it. And that they say is equivalent to 33 point 7 kilowatt hours of stored electricity. Kilowatt hours is typically how you measure hours in a battery. Now the math isn't simple, as you can imagine, and this is part of they actually pull that equation together. Let's take a look at how cars that use mpg are sorted out. Battery Electric cars can have mpg, like a Nissan Leaf or a Tesla Model S. All you do is plug them in and that's there only source of power. Then you've got plug in hybrids. Those are vehicles that have a gas engine and a bigger battery and can still run in a blended or separate modes. You've also got hydrogen fuel cell car, which is also electric, but using a different source to generate the volts. What you won't find in an MPG rating on is a conventional hybrid, because the EPA has just deemed those to be. Too conventional. Now, if you look at the sticker, it's an interesting story. Here's a battery electric car. All it has is an MPGe number cuz it doesn't run in any other mode. There's no gas involved. Go to a plug-in hybrid and it gets very busy. Here's your MPGe on the left, which shows the pure electric running. On the right, here's the blended system number that shows the fact that it also can run on gas. That's your MPG number. What I also want you to notice is what's in the fine print on these new stickers. Look underneath either the MPG or the MPGe, and you now find a European style gallons per 100 miles, or over here, kilowatt hours per 100 miles. This is not just semantics. If you do this, you can avoid what's called the MPG illusion, which basically says that if you measure a car that improves from ten to let's say 15 MPG. That is not the same as a car that improves from, let's say, 50 to 55. You might think those are equivalent gains. In fact, the lower number improvement is vastly more significant than the higher number improvement of the car that started at 50. But if you measure in energy per 100 miles, it's linear. The distortion is taken out mathematically. And of course the gas engine car doesn't get MPGE at all. You'll see your traditional number there on the sticker still because all it does is run on gas. More car tech demystified right now at CNETOnCars.com. Click on Car Tech 101.