When we talk about fuel economy in these parts we talk in miles per gallon, known to everyone as mpg. We may think we have it all figured out, but mpg is a stupid unit. It actually distorts the issue of fuel efficiency and leads to many misconceptions in the minds of consumers. We should all do ourselves a favor and stop using it.
Engineers know this. When they measure fuel consumption in the lab they talk in terms of gallons per mile. The CAFE standards that govern fuel economy targets and the EPA procedures that lead to window sticker numbers all operate behind the scenes in gallons per mile, too. The final results are only converted into mpg at the last minute for publication because consumers expect it.
What follows is why the engineers and the government rule makers actually have it figured out for a change. We as consumers should insist on the same sort of clarity and simplicity.
Efficient Cars Burn Less Gasoline
Ultimately, what most consumers really want to know is this: "How much money will I spend on gas?" For a given price of gasoline, that's the same as asking, "How many gallons will this car burn?" That's fuel consumption, and smaller numbers are better.
For some this question goes beyond spending less money and reducing dependence on imported oil because the number of gallons consumed also translates directly into CO2 emissions.
Gallons burned, therefore, is the quantity we want to measure, what your math teacher would call the "dependent variable." Said math whiz would go on to say that a dependent variable belongs on the top of the fraction. This means we should measure gallons per mile or gallons per hundred miles, not mpg.
Five Equals Two
Why is this important? Having gallons at the bottom of the fraction introduces a huge non-linearity. A single mpg has no fixed value; it isn't a tangible thing.
Take a car that is rated at 20 mpg combined: a 2014 Ford Mustang 5.0-liter V8 or a 2013 Hyundai Santa Fe, perhaps. After a week of less-than-patient driving, we fill it up to find we've gotten 18 mpg. We missed the rating by 2 mpg, which works out to 10 percent. Oh, well.
The next day we drive a 2013 Toyota Prius, which is rated at 50 mpg. And let's say we also underperform by 10 percent. Thing is, here that 10 percent works out to 5 mpg. But it's a hybrid. And it's off the mark by 5 mpg. That's outrageous!
Despite the difference in gut reaction, the 5 mpg miss in our Prius is no more significant than the 2 mpg miss in the Mustang. Five equals two. But people tend to think that the value of 1 mpg is fixed, that it has inherent meaning. Nope.
Actually, Ten Equals Two
Let's change both ratings into the gpm format, specifically gallons per 100 miles so the resulting numbers are greater than one.
Our 20-mpg Mustang consumes 5.0 gallons per 100 miles (gp100) and the 50-mpg Prius burns 2.0 gp100. Right away we can easily see how much gasoline each one will use over a given distance. And lower is better, which is how you want to think if your aim is to save money, use less fuel, emit less CO2 or all three.
Now let's see what happens when each underperforms by 10 percent as before. Starting at 5.0 gp100, the Mustang's 10 percent miss equates to an extra 0.5 gallon, bringing actual consumption up to 5.5 gp100. The same 10 percent offset brings the Prius' consumption up by 0.2 gallon to 2.2 gp100.
What this means is the Mustang's 2-mpg miss is actually more than twice as significant (0.5 gallon vs. 0.2 gallon) in pure gallon terms as the 5-mpg miss in the Prius over the same distance.
At a fuel cost of $4 per gallon, the Mustang's 2-mpg shortfall results in an extra 0.5 gallon and $2, but the Prius' 5-mpg miss represents just 0.2 gallon and 80 cents over the same 100-mile distance. Two is greater than five.
In fact, the 50-mpg Prius would have to underperform to the tune of 40 mpg, 10 mpg below expectations, to get to 2.5 gp100 where it'd burn the same extra half-gallon and have the same out-of-pocket impact as the 20-mpg Mustang does when it achieves 18 mpg. Ten equals two.
Hybrids Take It in the Shorts
This misdirection has everything to do with the bad-math nature of the upside-down mpg unit. And it gets worse the higher the mpg number gets.
And so hybrids (and some diesels) tend to get criticized for underperforming far more than conventional cars. Hybrid offerings from Honda, Toyota and, most recently, Ford have taken heat in the form of lawsuits and local news exposés for gaming the system in the test but falling short in the real world.
But a good deal of the grief comes from a lack of understanding about how mpg works (or, more to the point, doesn't work) in the high-mpg rarefied air where hybrids and diesels live. Few people get too upset when their Mustang gets 18 mpg instead of 20 mpg; it's only 2 mpg. But take away 5 or 10 mpg from a 50-mpg hybrid and the world spins off its axis.
If we look at both in gpm terms, the real story emerges and the "problem" disappears.
It's All in Your Head
Finally, there's a practical reason to like the gpm format.
Say we're planning a trip. Our 2014 Chevrolet Silverado 1500 pickup is good for 18 mpg and we're going to drive about 200 miles. How many gallons? How much will it cost? Long division is involved. We'll need a calculator.
If our truck was labeled in the gpm format, its consumption rating would be 5.6 gp100. Multiply by 2 to get 200 miles' worth of fuel: 11.2 gallons. Multiply that by the price of fuel to get dollars. Sure, a calculator still helps, but the approach is more straightforward. Multiply, then multiply. And you're already starting with gallons, the thing you're buying.
And it makes budgeting and car shopping simpler, too. Many of us drive about 1,000 miles each month. It's ridiculously easy to take a gp100 miles rating on a window sticker and multiply it by 10 to get the number of gallons used during that month. Our 5.6 gp100 Chevy pickup would go through 56 gallons each credit card cycle.
You say you drive 1,500 miles a month? OK, tack on another 28 gallons. You don't have to be Rain Man.
The gpm format removes all the confusion and lays bare the information we really wanted to know in the first place: How much fuel does this thing use? We really ought to be using a gpm-based unit in all discussions of fuel consumption. It should be on the window sticker.
Oh wait, it is. The EPA snuck in the gpm format when a new window sticker design was rolled out for the 2013 model year. Today a vehicle's combined gp100 rating is right there, just below the prominent combined mpg number. It's also listed on the fueleconomy.gov Web site.
They don't bother with city and highway versions of gp100, but that's fine with us because real-world driving is a combination of the two, anyway. The only thing more bogus than mpg itself is advertising that focuses strictly on highway mpg.
The gp100 rating on the new window stickers appears in small print, but the usefulness of the information it provides looms large. In actuality it's all you ought to be looking at.
For once, the fine print has it right. Find it, read it, use it, think it — and ignore MPG. It needs to disappear.