What is MPGe?

Filed Under: green automotive technology, Green Technology on July 23, 2010

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MPGe (Miles Per Gallon equivalent) is a measurement of a vehicle’s energy use. It’s meant as a replacement for the miles per gallon (MPG) we’re all familiar with. While measuring fuel use vs. miles driven is easily done by the average driver – and is often done automatically by many in-dash displays now – it is a measurement of efficiency that is fast becoming outdated.

Even with only combustion fuel alternatives, such as gasoline vs. ethanol or diesel vs. biodiesel, the comparisons can be apples-to-oranges. Ethanol has a lower energy content than gasoline, so it’s MPG measurement will be lower whereas biodiesel may have a different energy density or burn rate than petroleum-based diesel. This says nothing of when these fuel types are mixed, such as with most ethanol and biodiesel on the market – not to mention dieseline, a mix of gasoline and diesel that’s becoming popular as a way to improve the efficiency of some diesel engines.

Then when you throw in alternative cars like plug-in hybrids and all electric car options, MPG quickly becomes irrelevant. Even a gasoline-hybrid like the Toyota Prius or a range extended hybrid like the Chevy Volt cannot be reliably measured using pure MPG. This is especially true of vehicles like the Volt that run as an all electric car for at least part of their drive time (PHEVs).

The trouble with MPG is that it’s based on the question of “how far can I drive on this liquid fuel?” The new question to be asked of alternative vehicles is “how car can I drive on a fixed amount of energy?”

MPGe is one of the best solutions for that so far. It’s calculated like this:

MPGe = miles driven / [total energy of fuel(s) consumed / energy of a gallon of gasoline]

This means that if you drive 100 miles in your plug-in electric car and use the electrical equivalent of 6,826 BTUs (British thermal units, in this case 1kW of electricity for two hours) then you have gained an MPGe of 1,831.5:

(100 / [6,826/125,000]).*

This is a standard “pump-to-wheels” measurement of energy usage in a car. It’s what consumers are familiar with and what makes the most sense. Some would propose a “well-to-wheels” measurement, which accounts for the energy used to extract the fuel and move it to the location where the consumer put it into his or her car. That’s a more accurate measurement of the actual footprint of the energy, of course, but it’s something that is too complex to be done at the steering wheel for most consumers.

In addition, it can generally be assumed that this cost in energy has been accounted for at the pump by the price paid for the fuel in question. Anyone who pays attention to gasoline prices knows that they can change by several cents per gallon just by traveling ten or fifteen miles. This is usually a reflection of costs in transportation.

Continuing with our example of an electric car, someone with a garage who parks at home nightly and plugs in will probably have a cheaper recharge cost (say $1.00 as a base line) than someone who lives in an apartment and must pay for a “recharging meter” (a parking spot with an electrical meter that recharges at a rate of $1.25 for the refill). The extra quarter accounts for the costs of delivery (in this case the cost of installing a plug-service “pump” at a parking lot).

Several car companies are now working on MPGe and similar readouts for various types of EV cars. There are several creative solutions for the measurement problem including smart phone applications, in-dash connections for computers (USB or WiFi), and small screens built into the car that give this data directly. Collaborations with power utilities and other companies are allowing real-time dollar figures to be included in this data, so consumers can know exactly how much their recharge and drive cost when plugging in at home or the office.

While MPGe hasn’t been accepted by any regulatory agency, it is currently considered the standard for most vehicles using fuel alternatives. Many companies are publicizing their MPGe numbers along with standard MPG calculations for their alternative cars in order to promote better energy per mile results for their vehicles.

*One gallon of mid-grade gasoline has 125,000Btu. This calculation assumes 2 hours of driving (aka 50mph).