Calculating MPGe

Plug-in electric vehicles are poised to become practical and important, which is why we’re seeing a crescendo of reports about their “MPG” fuel economy.   Unfortunately, these reports raise more questions than they answer, including two questions that will occur to informed consumers:  “How is this ‘MPG’ value defined?”  and “What does it mean for me, given how I drive?”.  

In evaluating and comparing vehicles that are all powered only by gasoline, miles-per-gallon (MPG) makes great sense.   But MPG isn’t what it used to be.   As discussed in Eric Cahill’s recent post (here), MPG is obsolete for two main reasons: the growing popularity of alternative fuels, and the emergence of vehicles powered by multiple fuels (most importantly electricity plus gasoline or other liquid fuels). 

Comparing the MPG of gasoline with the MPG of ethanol (to use one example) is like the proverbial comparison of apples to oranges; besides, what’s a gallon of electricity?   For a Plug-in-Hybrid-Electric-Vehicle (PHEV), MPG is even more misleading (at best, it tells just part of the story – e.g., accounting for just the liquid fuel but not the electricity).  

As the figure of merit for fuel economy, the Progressive Insurance Automotive X PRIZE is using MPGe (miles per gallon energy equivalent), defined as: 

MPGe =  (miles driven) / [(total energy of all fuels consumed)/(energy of one gallon of gasoline)]) 

MPGe is a simple, well-defined measure of overall vehicle efficiency.   MPGe is easy to explain, accounts in a neutral manner for any combination of fuels, and reduces to the familiar MPG in the case of gasoline fuel only.

To make it easier to understand (and get some intuition about) MPGe, we’ve developed a spreadsheet available here.  It includes three simple calculators:

(1) Enter the distance driven and the amounts of all fuels consumed – result is MPGe.   The formula for this calculation is the one given above. 

(2) For the special case of gasoline-electric PHEVs, enter the gasoline usage (MPG gasoline) and electricity usage (watt-hours per mile) – result is MPGe.   The formula for this is given and discussed at length in my blog post available here.  

(3) For the case of any plug-in vehicle, enter the charging time and the charging efficiency – result, for three different electric circuit types, is the total energy added to the battery, expressed both as kWh and as (energy-equivalent) gallons of gasoline. 

This is a hard time for the automotive industry.   But it’s also an exciting time of change and innovation.   Consumers will soon have a variety of energy efficient and environmentally friendly vehicle choices as diverse technologies and alternative fuels come to market.   This diversity will make it harder to evaluate and compare vehicles, and how best to do so is a difficult question.   MPGe is not the entire answer, as it must be presented in the context of driving style, frequency, and distance.   But we believe it’s part of the answer.  MPGe should be the new MPG.  

Comments

[Jim Tarbell]

The choices do not include the most common fuel used in the USA today:

90% gasoline and 10% ethanol

[Don]

How about setting the new standard as gallons equivalent (Ge) per 100 miles? MPG is not intuitive. Most people cannot fathom that the benefit of moving from 15 mpg to 20 mpg (a savings of 1.7 gallons per 100 miles) is significantly greater than moving from 25 mpg to 30 mpg(a savings of 0.7 gallons per 100 miles). If efficiency were expressed in Ge per 100 miles all would become much clearer.

[drklaus]

Please see my article "GM's MPG Wonder, which will appear shortly on the web.
In it I propose to compare energy costs for driving exclusively on electricity or gasoline. We need to define two driving conditions; one for city driving, the other one for highway driving.
Each consumer is faced with a different set of individual driving conditions. Consumers need to foretell approximate driving costs of competing hybrids.

[david]

Why not calculate the cost/100 miles? Peo

[John, Redding Ca]

Personally I prefer cost/mi, with all the correct variables input its hard to beat. and it accounts for such things as vehicle cost, depreciation, battery cost, maintenance cost, insurance, license fees and taxe as well as fuel cost and is a better decision making tool than anything else. Like the X-prize spreadsheet all of the variables need to be explicitly stated to prevent the results from being skewed such as the difference between imperial gallon/mi and US gallon/mi

[Jeremy]

I think there is a severe flaw in the mpge. I have created my own spreadsheet prior to this marketing hype by GM, and I include the efficiency of the electrical grid, widely accepted as approximately 33%.

For example, the 170 mpge Volt (40 miles on 8 kwhr) would really be about 50 mpge.

I believe cost per mile is useful for the economics part for any given day. But consider how that value is vastly different from last summer, electricity rates are about the same, but gasoline is 40% cheaper.

Costs of fuels change and are subject to different levels of taxes. Fuels and energy values, however, remain constant forever.

[alfweinz]

I'm not an US citicen an I hate this MPG or MPGe.
When will you come to international standards? The only correct measurement is kWh (/100km).

[carl]

I agree with previous poster, kWh/100 km is a good standard if a new standard is needed. It will help
US industry compete better to follow global standards. Clearly protectionism with propriatary standards has not served the US or the auto industry well in the long run.

[Jim Bullis, Miastrada Company]

No matter how many times it is asserted, electricity is still not a fuel. It is a carrier of energy from a point where fuel is burned.

The outcome of this wrong assumption is that electricaly propelled operation is given a three to one advantage over operation driven by fuel burned in the car.

This means that a trivial amount of innovation will qualify for the 100 MPGe requirement and much effort and sacrifice of vehicle performance is required for vehicles that produce energy from fuel.

This is a serious flaw in the XPrize competition. It will continue the widespread but false pretense that coal is not the fuel that will be used to respond to the incremental load of each electric car.

[Balazs]

If you want to factor in production and transport costs for electricity, then you should start with the coal mine not the power station. Similarly you could start considering how much petrol the fuel carrier tanker truck consumes.
This would make stuff rather complicated.
Also if I co-generate electricity with my own wind turbine will that make my car more energy-efficient?

[Jeremy]

Balazs,

Of course starting at the coal mine and likewise the oil rig (or fuel carrier tanker) would be more accurate and complicated.

X-prize calculation is actually good at simplifying energy use per mile or gallon equivalent. However it is severely flawed because it ignores fuel consumption during electricity generation.

From an overall perspective, it is a similar and minor (when looking at total energy %) omission if we were to look at the fuel consumed at the power plant and gas purchased at the gas station.

Another way of looking at it: being wrong by 300% for one type of energy conversion is a lot worse than being wrong by 10% for both types of conversions.

[Jim Bullis, Miastrada Company]

Electricity is a carrier of energy. It acts like a drive train of a car. If the axle was connected to a paddle that stirred water, the water could be made to increase in temperature. If electricity was converted in a resistance in that water, it could also cause the water to increase in temperature.

If the the same kWhrs of mechanical energy and electric energy were used to heat a tank of water the temperature increase would be the same in either case.

Everyone is aware of the heat engine that turns the driveshaft of the car. There is a corresponding heat engine turning the electric generator in the central power plant. By far the most important efficiency issue is at the heat engine, in either case. It can not reasonably be excluded from the comparison for one form of propulsion and not the other.

Making the right decisions is a matter of national importance, and it looks like Xprize has no concern about the truth in this regard.

[Ashutosh]

Another complication is to consider the source of electricity itself. The fuel consumed can vary widely, starting from NIL, for electricity generated using wind, solar, hydel, coal, gas, or nuclear.

Perhaps a better measure is to look at end-to-end CO2 emission - as ultimately that is what is the driver for the efforts being put into developing alternate fuels for automobiles.

[Ken Fry, GaiaTransport]

Jim Bullis is completely correct. There is no logic or truth to the assertion that electricity is a fuel. Fuzzy thinking on this can lead to bad policy, bad buying decisions, and bad environmental results.

Anyone who claims that electricity is a fuel needs to open up a chemistry text, a physics text, or a simple English dictionary.

Fuels burn. Fuels have mass. Fuels are depleted: we cannot easily change the CO2 from burning coal back into coal. When we burn coal to generate electricity, we deplete a particular mass of coal and create a particular mass of CO2. No amount of spin will change that fact.

Re the calculator, why is there a "special case" for PHEVs? (Case 2 above.) If the instructions are followed, and the Volt figures of 250 watt-hours per mile and 40 miles per gallon are used, the MPGe result is 30.9 MPGe. Who will believe such a thing?

Even the pre-primed numbers that are there (in the "Convert PHEV MPG and WH/MI to MPGe" section) when you download the calculator are ludicrous. How can a 150 mpg vehicle (X Prize MPGe: 150) which uses 200 watt-hours per mile (X Prize MPGe: 136) get 79.7 MPGe??? A logical person would expect a figure between 136 (elec only) and 150 (gas only).

The underlying formula is wrong. The fuzzy thinking on electricity is wrong.

Endlessly repeating "Electricity is a fuel" will not make it so. Science does not work that way -- it's not a popularity contest.

A goal of this competition is (ostensibly) to educate the public. Promoting an anti-scientific, anti-engineering view is not education.

There is much at stake here. The world deserves better than flawed concepts and flawed formulas.

[J Z]

Whoever thought up the 'Calculator' has to be in sales.

[Tim Wieck]

Why does PROGRESSIVE use 116,090 BTU/gal for their MPGe calculation?
It would seem that 113,500 would be a better average energy density per
http://www.epa.gov/otaq/rfgecon.htm#1

By the way, I agree that energy Usage per Distance (kWh/100km)is a better concept than Distance per Volume (mpg)
It encourages conserving.

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