Thanks to my friend Dan in California for posing a question about my Tesla Supercharger story last week. He asked, “So how do youn calculate mpg when there is no G?” Good question. The following is from the window sticker of my Model S.
The government calls it MPGe, with the e standing for equivalent. I don’t know how they calculated the 102 mpg, but I will share with you how I would calculate it. Somewhere along the line, probably in engineering school 50 years ago, I learned a technique called Dimensional Analysis. It has been a handy tool for me all these years. The purpose of the tool is to convert one unit of measure to another. The method is simple. You multiply fractional representations together. Below is what I scratched out on a piece of paper.
When you multiply miles per kWh times kWh per $ times $ per gallon, you get miles per gallon. Voila. Now lets add some numbers. The first fraction is miles per kWh. The Model S has a range of 335 miles with a full charge, which is 100 kWh, so that fraction is 3.35. The second fraction reflects the cost of one kWh. The cost varies greatly from state to state. The Choose Energy website shows the average price by state. It ranges from $.093 per kWh in Louisiana to $.223 per kWh in Rhode Island. (I excluded Hawaii and Alaska as they are off the charts). The third fraction reflects the cost per gallon of premium gasoline. This cost, as shown on the AAA website, varies greatly also, from $2.37 in Missouri to $3.64 in California.
Now, just multiply the three factors together to get the miles per gallon equivalent. For the high side, lets use Louisiana electricity and California gasoline. The result is 132 miles per gallon. At the other extreme, lets use Rhode Island electricity and Missouri gasoline. The result is 36 miles per gallon. Reality is somewhere in between. If half of the electricity you use for charging is from a free Tesla Supercharger, the results easily go above 200 miles per gallon.