That’s right, MPG is bullshit (and along with it, MPGe).
I don’t mean that the emphasis on improving fuel efficiency in personal transportation is bullshit, to the contrary, it’s one of the most important things we can do today. Similarly, I see the value in representing the efficiency of non-gasoline vehicles in a way that translates to gasoline units of energy – it helps put things into context for an efficiency-minded buyer.
The problem with MPG is that it, as a metric for measuring vehicle efficiency, is terrible at representing that efficiency in an intuitive way, and as a result, has discouraged improvements in efficiency in the vehicles that need it the most, and caused efforts to be directed to vehicles that need it less. To illustrate this, I’ll create a scenario.
You have two older vehicles that you drive about equally, one that gets 15 MPG (probably a full-size pickup or a large SUV), and one that gets 30 MPG (probably a compact car). You’ve got the funds to replace one vehicle, and you want to get something more fuel efficient, without losing capability – so you’re looking at either another full-size pickup or SUV, or another compact car. You look at the vehicles that are available, and see that you can get a full-size truck or large SUV that gets about 20 MPG. Alternately, you can get a car that gets 50 MPG nowadays, and it’s even a fair bit bigger than a compact. Which should you buy, to reduce your fuel consumption by the most?
Intuitively, you’d get the car – it gets 20 MPG better than your car, and it’s 67% better than your existing car on MPG. The truck only gets 5 MPG better, and only 33% better MPG.
And this gets into why MPG is bullshit – MPG determines how far you go on a fixed amount of fuel, and you’re not driving for a fixed amount of gallons, you’re driving a fixed amount of miles. In Europe, the standard (at least outside of the UK, anyway) is to report fuel economy in terms of liters per 100 kilometers. It answers the question of how much fuel it takes to go a fixed distance, instead of how far you can go on fixed fuel. Metric system issues aside, I’ll illustrate how this is a superior system for representing efficiency, using gallons per 100 miles – the familiar units in the US.
Under the gallons per 100 miles system, your truck is now rated for 6.67 gal/100 mi, and your car is now rated for 3.33 gal/100 mi – the conversion is merely 100 divided by the MPG. And, the new truck and new car are rated for 5 gal/100 mi and 2 gal/100 mi respectively. So, in 100 miles, the new truck uses 1.67 gallons less fuel over 100 miles, whereas the car only uses 1.33 gallons less fuel over the same distance. Upgrading the truck reduces your fuel consumption more than upgrading the car, even though the intuitive ways of looking at MPG (numeric or even percentage improvements in MPG) make it look like the car is the better option.
Ultimately, because MPG as a measurement is relatively insensitive to even large improvements in efficiency in inefficient vehicles, while magnifying minor improvements in efficiency in already efficient vehicles, it’s arguably hurt the American automotive marketplace. The American automotive market is one that buys plenty of large, inefficient vehicles for various reasons, and in those vehicles, if a consumer sees a “mere” 1 or 2 MPG difference between two models, they may be less inclined to take a more efficient option, even though it would save a significant amount of fuel. Conversely, consumers may prioritize replacing already efficient vehicles with vehicles that are only slightly more efficient, because of a large difference in MPG.
It’s worth noting that on fueleconomy.gov, the US government’s website for information on vehicle fuel economy, in addition to the MPG (or, for electric vehicles, the MPGe) figures, they list gal/100 mi and kWh/100 mi figures (in smaller print, however, leading with MPG or MPGe), as well as allowing users of the site to have figures displayed in either gal/100 mi or l/100 km. I applaud them for this much, but I’d personally like to see MPG abolished altogether, in favor of reporting efficiency in gal/100 mi (l/100 km is just asking too much, especially because fuel’s sold in gallons and distance is measured in miles in this country) as the primary method of reporting for liquid fueled vehicles, as well as on the Monroney sticker that’s on all new cars.