popped up on Twitter last Tuesday, but didn't come to my attention until yesterday:
Nissan Leaf = 367 mpg, no tailpipe, and no gas required. Oh yeah, and it'll be affordable too!
This message was obviously aimed right at the stomach of Chevy Volt's announcement of a 230 MPG rating from the DOE. Unfortunately it missed, and instead went right into the logic center of my brain, causing it to rattle and spark. This cannot be true, I thought.
Truth begins to reveal itself in the weasel wording of the next message
from Nissan, posted about 90 minutes later:
To clarify our previous tweet, the DOE formula estimates 367mpg for Nissan LEAF.
Which begs the question: what is the DOE formula for estimating miles per gallon for an electric vehicle?
If you are thinking you are not interested in reading this, or think its going to be too long or boring, I cannot stress enough how much you should finish this, and how upset you'll be when you are done. Think of this like an M. Night Shyamalan movie: its going to have a twist ending that will leave you confused, angry, and possibly both.
First, let's briefly review my method of evaluating the efficiency of electric vehicles, which I defined as MPGe, or miles per gallon of electrons. A gallon of electrons is the amount of electricity equal to the energy in a gallon of gasoline. For details on the methodology, click here
The Nissan Leaf boasts a range of 100 miles on an electric charge of 24 kWh. This breaks down to 4.2 miles/kWh, or 150 MPGe (multiply the miles/kWh by 36.6 kWh/gallon, the energy density of gasoline, to get MPGe).This article
from the Society of Automotive Engineers details the methodology taken by the DOE to get to their MPG rating. I had to read it 5 times before I finally understood what was going on, so I won't blame you if you get lost. Let's just go step by step.
STEP ONE: Determine the miles per kWh for the vehicle. We've determined that is 4.2 miles/kWh
STEP TWO: Convert to gallons using the energy density of gasoline. (Note that so far my methodology is identical). The DOE uses an energy density of 33.7 kWh/gallon instead of 36.6. I don't take it personally. 4.2 miles/kWh x 33.7 kWh/gallon = 141.54 miles per gallon
STEP THREE: The DOE takes into account transmission and production efficiencies, not just usage at the vehicle. I had thought about using this in my evaluation as well, but I decided that I would draw the line at the place of purchase instead of production (as far as I knew, current MPG ratings do not take into account transmission or production energy that goes into making gasoline - we'll get to that in a second). The transmission efficiency used is 96%, and the production efficiency is 32.8%. This is the efficiency of a typical coal plant, and so can vary significantly depending on where you get your electricity, but WHATEV, let's just play along. 141.54 miles per gallon x 96% x 32.8% = 44.6 miles per gallon.
STEP FOUR: Here's where I learned something: CAFE standards actually DO take into account the distribution and production of petroleum. They use an efficiency factor of 83%. Because the electric vehicle does not use any petroleum, the MPG rating gets a boost. 44.6 miles per gallon / 83% = 53.7 miles per gallon.
STEP FIVE: Multiply by 6.67 for no apparent reason. 53.7 miles per gallon x 6.67 = 358 miles per gallon
. Uh... huh? Why?
The Corporate Average Fuel Economy, or CAFE standards, are a set of rules which legislate the average efficiency of a fleet of vehicles made by a given manufacturer. The overview of the rules is here
. The relevant passage of the rules is reproduced here for your enjoyment:
The CAFE law provides for special treatment of vehicle fuel
economy calculations for dedicated alternative fuel vehicles
and dual-fuel vehicles. The fuel economy of a dedicated alternative
fuel vehicle is determined by dividing its fuel economy in equivalent
miles per gallon of gasoline or diesel fuel by 0.15. Thus a
15 mpg dedicated alternative fuel vehicle would be rated as
100 mpg.53.7 miles per gallon / 0.15 = 358 miles per gallon!!!!
Just so we are clear, I will repeat that it has absolutely nothing to do with the vehicle's utilization of energy.
I didn't get 367 - the 4.2 miles/kWh I started with was an estimate, not the actual results of DOE's testing - but this is the method used to calculate the fuel efficiency of alternative fuel vehicles. This is not just for electric cars. Flex fuel vehicles that use ethanol use this same fuzzy math when they publish their fuel efficiencies. The lying isn't new, its just getting more noticeable.
Why is this allowed? The justification is that it encourages manufacturers to make non-petroleum vehicles by giving a bonus to their CAFE number. As CAFE standards increase, the manufacturer has the option of either making their vehicles more efficient, or abandoning petroleum as a fuel source.
I can live with that. But now those numbers are being used to deceive the public. People hear 367, or 250, and nobody understands. Nobody knows what it means. Except you, dear reader. You understand.
And also understand this: I am not angry and writing all this because I want electric vehicles to die. Its exactly the opposite: I think electric vehicles are the future. By creating unrealistic expectations for the performance of their vehicles, Nissan, Chevy, and everyone else who publishes this garbage are setting themselves up for failure.