MPG Is Stupid

[heleu]

MPG Is Stupid on Edmunds.com

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.

MPG Is Stupid

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.

MPG Is Stupid

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.

MPG Is Stupid

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.

GPM FTW
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.

Simple concept, but it is a big difference.

Canadians have a metric unit (L/100km) FTW for once.

[smoothie.]

For once? You mean for everything.
Posted via RS Mobile

[nsx042003]

what? why does it matter? mpg or pgm, it's just a reference to show how much fuel a car uses to move a certain distance. As long as you don't cross reference (ie: mpg vs pgm), I don't see the point of saying which metric is better.

[UFO]

I personally like to think in L/km myself as L/100km doesn't practically mean much to me. I can calculate how much an 18km trip will cost me in terms of fuel consumption. My personal spreadsheets I like to calculate my $/km driven factoring maintenance and insurance, could easily use that to compare with whether taking public transit is a considerable cost vs. time savings.

[Traum]

Originally Posted by nsx042003

what? why does it matter? mpg or pgm, it's just a reference to show how much fuel a car uses to move a certain distance. As long as you don't cross reference (ie: mpg vs pgm), I don't see the point of saying which metric is better.

Please read the article again. It has clearly explained why the mpg metric is misleading.

[4444]

I think that article is for those with IQ's under 100 - mpg is what it is, if one can't analyze and compare it fairly with some intelligence, then that's ur bad.

It's a tool, a bad workman blames his tools - this article just supports that

[Ferra]

Originally Posted by nsx042003

what? why does it matter? mpg or pgm, it's just a reference to show how much fuel a car uses to move a certain distance. As long as you don't cross reference (ie: mpg vs pgm), I don't see the point of saying which metric is better.

because mpg is generally a non-linear expression...which makes comparison difficult & confusing especially when you start looking at the more extreme ends..

Example: when you compare 50mpg vs 70mpg...the 20mpg difference looks huge
However, when you do the calculation, that 20mpg difference will only save you 1.3L of gas over a 100km trip.

On the other hand, when you compare 10mpg vs 30mpg (also 20mpg difference),
this 20mpg difference will now save you 15.7L of gas over the same 100km trip...

[heleu]

Thank you. I'm glad someone here actually read the article!

[4444]

Originally Posted by Ferra

because mpg is generally a non-linear expression...which makes comparison difficult & confusing especially when you start looking at the more extreme ends..

Example: when you compare 50mpg vs 70mpg...the 20mpg difference looks huge
However, when you do the calculation, that 20mpg difference will only save you 1.3L of gas over a 100km trip.

On the other hand, when you compare 10mpg vs 30mpg (also 20mpg difference),
this 20mpg difference will now save you 15.7L of gas over the same 100km trip...

You're 100% right - but is this something new? Don't ppl already know this?

[Ferra]

Originally Posted by 4444

I think that article is for those with IQ's under 100 - mpg is what it is, if one can't analyze and compare it fairly with some intelligence, then that's ur bad.

It's a tool, a bad workman blames his tools - this article just supports that

I think the article point is...mpg is an "inconvenient" tool
Most people think in-terms of the distance they need to travel...(i.e. km or miles), and not how many gallons or liter they need to use..
This means the distance should be in the denominator...(becoz people are more used to multiplication than division)

MPG uses the distance in the numerator and gas volume in the denominators....which makes calculating how much gas/$ you need to spend to in order to travel your annual 20,000 much more difficult.

Think of a supermarket that list all their groceries in lb/$ (instead of $/lb). It will make shopping and budgeting for prices so much more difficult. (Because we think of how many lbs of beef we want to buy first, instead of how much $$ worth of beef we want to buy)

[4444]

Originally Posted by heleu

Thank you. I'm glad someone here actually read the article!

I read it too, thanks, I agree with what ppl are saying - doesn't mean it's stupid, it's just a tool to use. I'm sorry if ppl are so simple to not realize the difference between a 20mpg difference at the low end and high end.

Again, it's just a tool

[4444]

Originally Posted by Ferra

I think the article point is...mpg is an "inconvenient" tool
Most people think in-terms of the distance they need to travel...(i.e. km or miles), and not how many gallons or liter they need to use..
This means the distance should be in the denominator...(becoz people are more used to multiplication than division)

MPG uses the distance in the numerator and gas volume in the denominators....which makes calculating how much gas/$ you need to spend to in order to travel your annual 20,000 much more difficult.

Think of a supermarket that list all their groceries in lb/$ (instead of $/lb). It will make shopping and budgeting for prices so much more difficult. (Because we think of how many lbs of beef we want to buy first, instead of how much $$ worth of beef we want to buy)

I think that's a good analogy, my counter would be, I don't care if its $/lb or lb/$, what I care about is the difference week on week, or shop to shop, it's a comparison, as opposed to an absolute calculation tool

[Simplex123]

Yes you're right, on a day to day basis it does not make a big difference. But for people who do computations to keep records and compare data, the 20mpg as Ferra pointed out would mean nothing as it is not linear - forcing them to take an extra step and convert to a linear unit.

[nsx042003]

Edmund should've wrote something on how EPA define what is "highway" driving vs "city" driving. I think that would've been more interesting than what this article is saying.

I just don't think mpg is anymore wrong than GPM or GP1000 or GP10000. It's just a figure that answers different types of questions. Because quite honestly, anyone can convert the numbers to whatever they find suitable for their calculation and whatnot.

[Gridlock]

Originally Posted by 4444

You're 100% right - but is this something new? Don't ppl already know this?

Yes, but any system that isn't straight forward can be rigged by marketers.

And most people don't want to sit in a dealership and do linear regression to get a simple answer to a simple question.

For most people, they want a car that is comfortable and a color they like.

"Is this fuel efficient?"

"Yes"

"Sold!"

Hell, I wouldn't sit there and automatically know that you can't directly compare the 2 MPG ratings.

[unit]

avi ftw

[Geoc]

Originally Posted by Ferra

because mpg is generally a non-linear expression...which makes comparison difficult & confusing especially when you start looking at the more extreme ends..

Example: when you compare 50mpg vs 70mpg...the 20mpg difference looks huge
However, when you do the calculation, that 20mpg difference will only save you 1.3L of gas over a 100km trip.

On the other hand, when you compare 10mpg vs 30mpg (also 20mpg difference),
this 20mpg difference will now save you 15.7L of gas over the same 100km trip...

I like how Ferra's little post said the exact same thing as the article in clearer way without writing a whole damn essay.

[4444]

Originally Posted by Gridlock

Yes, but any system that isn't straight forward can be rigged by marketers.

And most people don't want to sit in a dealership and do linear regression to get a simple answer to a simple question.

For most people, they want a car that is comfortable and a color they like.

"Is this fuel efficient?"

"Yes"

"Sold!"

Hell, I wouldn't sit there and automatically know that you can't directly compare the 2 MPG ratings.

i don't really think ppl need to do any kind of real analysis when it comes to cars.

what do average 200hp cars get these days, 35mpg? something like that -to me i'd just think, ok is this 200hp car i'm buying 35mpg or +/- a couple, we're good to go - if it's 25 mpg, why? i think mpg is more of a relative thing to what you're expecting as opposed to actually sitting there and calculating the dollars and cents of fuel consumption.

the reason i like mpg is that england uses MPG (i'm from there) - US uses mpg (although they use US gallons, but that's a minor adjustment, 35mpg UK = about 30mpg US or so), but canada uses l/100km... i can't compare apples to apples here. so i know that 8l/100km is about 35 mpg or around that. it's all about relative markers.

this is all moot anyway, as we never really get the advertised economy anyways

[hk20000]

Originally Posted by 4444

i don't really think ppl need to do any kind of real analysis when it comes to cars.

what do average 200hp cars get these days, 35mpg? something like that -to me i'd just think, ok is this 200hp car i'm buying 35mpg or +/- a couple, we're good to go - if it's 25 mpg, why? i think mpg is more of a relative thing to what you're expecting as opposed to actually sitting there and calculating the dollars and cents of fuel consumption.

Average high 200HP cars have a very very vast rate of fuel economy.

Let's say Camry V6 vs BMW 328i. Good luck assuming they use the same amount of fuel just because they all make almost 300HP and are about the same size and both are sedans. Then you throw in stuff like SUVs and trucks and light weight sports cars that also make the same power, and your argument is completely flawed. Way too many factors other than gross output determines the fuel economy.

on the avg MPG the Camry reads 25mpg and the 328i reads 20mpg, so you think they are both like 20 ish mpg. Then in gp100 it's 4.0 vs 5.0.

So during the ownership of the car, if both vehicles drove 100 thousand miles in its useful lifespan, you are buying another 1000 gallons of gas to get you the same A to B and at today's rate it's another 5000 plus dollars considering that the BMW uses premium gas.

That is what really goes into the buying decision for many engineering / math / accounting background sophisticated buyers.

[dared3vil0]

^ 328's only make 240hp brah.

[hk20000]

damn it's less powerful than a Camry! And use more gas...ffuuuu

well a different year let's say it's a 330i making 276HP then. Similar mpg to the 328i.

[Soundy]

Originally Posted by Ferra

because mpg is generally a non-linear expression...which makes comparison difficult & confusing especially when you start looking at the more extreme ends..

Example: when you compare 50mpg vs 70mpg...the 20mpg difference looks huge
However, when you do the calculation, that 20mpg difference will only save you 1.3L of gas over a 100km trip.

On the other hand, when you compare 10mpg vs 30mpg (also 20mpg difference),
this 20mpg difference will now save you 15.7L of gas over the same 100km trip...

Regardless of how the two are PERCEIVED, mathematically mpg and gpm are direct inverses of each other and the relationship between the two will always be the same.

The real problem is, people aren't taught to THINK anymore, so they have to have information like this spoon-fed to them like they're mindless idiots.

[hk20000]

^ true that, but in terms of what I demonstrated if you are really penny pinching you could easily pull out how much more money the worse fuel economy car is going to burn through during the ownership period.

Or in the scenario where you consider trading in your gas guzzler for a fuel efficient new car - how much are you paying now vs how much are you going to save for the life of the car.

mpg is more like just a figure until you do the maths. It's as meaningful to anyone as how you rate chicks an 8 out of 10 - but a 1 chick is disfigured compared to a 2 chick, yet you are probably going to bang an 8 just as readily as a 9 chick. gp100 is useful like "that watermelon is 8lbs" and the price is $0.5 per pound, then you know the watermelon is $4 for your summer enjoyment right away.

Now I want a watermelon. Or a 9 chick. Or both.

[skyxx]

Doesn't matter. The Gasoline engine is only at best(Modern engines) 30% efficient in "processing/Burning" the fuel.

[hk20000]

But the oil and gas companies are 100% efficient in taking your money from your wallet when you fill up.