Thoughts on how we communicate vehicle efficiency

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”

3 Responses

  1. Kevin,

    Defining the task would help derive the unit of measure. The task of a vehicle is to move something from point a, to point b, in an acceptable amount of time. We are looking to measure how efficiently a vehicle completes the task of moving cargo from point a, to point b. We would need to decide units for energy watts, mass kilogram, and distance meter. Something like milliwatts/ kilogram meter example all vehicle should be measured at their gross vehicle weight. Example Prius 259milliwatts/kilogram meter F150 350milliwatts/kilogram meter. So a Prius is 1.3 times more efficient than an F150 pickup.

  2. The only problem I see with cost comparisons of vehicles using different fuels is the same message that I sent to Craig and is the reason why we must also use some standard measurement of energy consumption in addition to cost. The problem is that the cost of fuels change, especially in comparison to each other in different seasons and in different markets, and sometimes the fuels themselves change, at least the liquid kind does from place to place and from season to season. The example I used with Craig is diesel fuel. In the winter, when home heating fuel is in highest demand, this is also the time when diesel is likely to be the least energy dense, yet it is also the time it is likely to be the most expensive compared to gasoline. In the summer, when diesel fuel is the most energy dense, it is then peak driving season; gasoline goes up; and diesel may sometimes drop down to an equal price. If we compare cost of fuel for two vehicles using two different fuels going the same distance at the same speed at the same approximate time, on the same route, it’s simply a snap shot comparison, but energy consumption lets us use a more constant measurement than varying costs.

    It seems to me that some people have done an awful amount of work to come up with gasoline gallon equivalents or GGE, using gasoline base units for comparison for everything from summer blend gasoline, to methanol, to KWh, and therefore, I think it’s a good comparison tool, especially since it’s what most of us can best relate to regarding transportation energy consumption.

  3. Energy use in an EV, is measured the same as it is at home: kWh, by the day and by the month or two, on the bill, but we don’t talk about home efficiency, except how big the bill was.

    We could say – “My house uses 25 kWh a day and goes nowhere, but my car uses 25 kWh and carries me ~100 miles!” Which is more efficient?

    Or we could say, “My car used 16 kWh to get me to work and back, some 60 miles, that’s like about a half gallon of gas in terms of energy used, which would be like getting 120 Mpg, and it cost me just $2.00!” (And the next day, it was full again when I left the house!)

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