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The advent of plug in vehicles, whether hybrids or fully electric, signal the end for conventional miles per gallon ratings. The electrical energy in the battery needs to be taken into account and therefore some kind of conversion between electrical energy and gallons needs to be made. Do you calculate the amount of energy stored in the gas and the battery and compare? But what about the energy that went into producing the electricity, which is a highly organized form of energy, do you include that? If so, what about the energy that went into refining the gas and transporting it? Such a calculation becomes so complicated and opinionated that ultimately it comes down to what the NY Times describe as ‘fudge factors’.

This is just an off hand idea, but what about a simple miles per $ measurement to compare plug in hybrids? You calculate how much it costs to fill the tank and charge the battery – perhaps using national averages for gas/electricity prices – then drive it until all the juice is gone and see how far you get.

Now, I know this is flawed. Gas/Electricity prices fluctuate for one, nor can you derive what ratio of gas to electricity a car uses based on a miles per $ figure, but there’s no methodology that doesn’t have problems. Also traditional mpg figures are always quite different to real world usage, the important thing is just that they are standardized so you can compare different vehicles directly. So as long as you use the same prices to calculate MP$ figures for different cars, you will have a fair comparison.

Without dwelling on the flaws, let me point out two simple benefits: Firstly and most importantly, there is no opinion in the calculation and there can be no gaming. It is entirely objective. As the NYT says in their article, the current ways to calculate miles per gallon-equivalent (MPGe) are ‘no longer a strictly scientific measurement, but takes into account compensating factors’. This subjectivity is always going to be problematic as you will never be able to satisfy all the different manufacturers, environmental groups & scientists with the compensating factors that you choose and you run the risk that companies with most influence will get factors and comparisons most suited to their vehicles. With a measure of miles per $, there is no scope for this kind of wrangling and opinion.

Secondly, it caters to the average car buyer’s main concern: price. Consumers will get a much better idea of which cars will offer the lowest running costs. Conveniently, lower running costs are highly correlated with fuel economy, so although consumers might simply be trying to save money the end result will be a reduction in their carbon footprint. You could even speculate that because consumers react more strongly to price than any other concern, a MP$ metric might actually induce even more people to purchase vehicles with greater fuel economy than if they were doing it simply for the green benefits.

As I said previously, this is just an idea that I accept is not perfect and I welcome any feedback in the comments. In a world where corporate transparency is very blurred and traditional mpg estimates seem so far from the real world performance of vehicles, I believe that MP$ could prove a more accurate, honest and relevant statistic for consumers. The EPA is working on a methodology for MPGe as we speak, and from what I understand they are not considering anything like this, rather there will be some kind of system that converts the electrical energy into an amount of traditional petroleum gallons.