I’m often asked why MPG or MPGe is used when discussing the amount of energy used as fuel. Most recently I had a conversation with Craig Vetter about this and it seemed worth reproducing my thoughts here:
I think the way we measure energy use for vehicles is flawed. Why do we use the unit of a gallon, a unit of volume, to represent energy or its efficient or inefficient use. When, for example, building a power plant it is not designed, described or defined by the amount of train cars of coal it consumes….you never hear of a 40 car coal plant being built or of a plant rated at 40 coal cars per day. We use a power rating, generally megawatts, for a commercial power plant. So we end up with power plants rated, for example, at five or fifty megawatts.
So why do we use the equivalent coal car rating system, gallons of gas, when we speak of motorcycles and cars? It doesn’t make a lot of sense, just because something’s always been done that way, because we have long bought automotive fuel in gallon units, and the average person has a vague idea of how far a gallon of gas will take them is not a sufficient reason to continue the practice. From this flawed practice, to expand upon it in the hopes of making energy consumption comparisons easier, leading to the mile per gallon equivalency comparison being born. It’s a questionable undertaking at best: volumetric comparison to another common volumetric measurement that is used to represent a unit of energy, when the actual content and volume of that ‘gallon’ varies based on environmental conditions, time of year, manufacturer, and location on the globe.
That said, it seems whether the use of either MPG or MPGe as a measure of energy consumption seems reasonable or not, most people are able to easily see the difference in energy consumption between two vehicles when using the MPGe standard.
Should either of them continue to be used moving forward? Does their use actually hamper our adoption of more efficient means of transport? Do any of the current methods used, mpg, cost per mile, mpge, or parsecs adequately fill the need of an easily understandable unit of measure to aid us in making truly informed decisions when it comes to energy consumption? Do they take into consideration not only transport but environmental and social cost, sustainability, and infrastructure needs? Would any unit or simple combination thereof accomplish this goal? Maybe, but probably not in an easy, straight forward manner.
However, we still need some way of making these comparisons. Moving forward, I would propose a two unit method for measuring cost and energy consumption. The first would be a unit of currency and it would measure average cost per mile, and the second would be a unit of energy to measure average energy usage per mile. Since dollars are convenient and a well known and measured unit of currency, I propose, at least for the States, that we adopt the first unit of measure as dollars or cents/mile. And for energy, since one of the most common units of energy world wide is Watts or kilowatts, I propose for our energy measurement kilowatt hours/mile.
With those two units we can determine out of pocket cost and the actual energy consumption/environmental impact of the energy consumed for any fuel type or mix consumed.
So, to keep things as accurate and simple as possible:
Also a possible point of note: busses are only an efficient means of transport because they move multiple people at once, and the whole point of transportation is not moving the vehicle but the people and goods. So in addition to cost and energy consumption on a per mile basis, incorporating a per person or per unit of goods into the cost and energy use equations could give a more accurate representation of the overall unit costs associated with any trip or delivery.
Those are my basic thoughts for now, any feedback would be welcome. It’s not always easy to see all sides of any problem and come up with a simple solution. But if we all put our heads together and hash out some ideas back and forth; we are the builders and racers, the ones on the cusp of this problem after all, I’m sure we’ll come up with something that’ll be copied by other efficiency challenges and have people saying to themselves “why didn’t I think of that”