2 In 3 Misunderstand Gas Mileage; Here's Why

thecarchik sends in this piece, which was published last March but remains timely: "OK, so here's a little test: Which saves more gasoline, going from 10 to 20 mpg, or going from 33 to 50 mpg? If you're like most Americans, you picked the second one. But, in fact, that's exactly backwards. Over any given mileage, replacing a 10-mpg vehicle with one that gets 20 mpg saves five times the gasoline that replacing a 33-mpg vehicle with one that gets 50 does. Last summer, Duke University's Fuqua School of Business released a study that shows how much damage comes from using MPG instead of consumption to measure how green a car is. Management professors Richard Larick and Jack Soll's experiments proved that consumers thought fuel consumption was cut at an even rate as mileage increased."

Comments

3 people in 2 don't know math.

[Tumbleweed]

News at 11!

Re:3 people in 2 don't know math.

[nmg196]

> 3 people in 2 don't know math

Wow that's scary. That's nearly half!

Re:3 people in 2 don't know math.

[Imagix]

There's 10 types of people in the world, those who understand trinary, those who don't, and those who mistake it for binary.

Re:3 people in 2 don't know math.

[Dragonslicer]

> 3 people in 2 don't know math

Wow that's scary. That's nearly half!

Seriously? Are you really that stupid? 3 divided by 2 is 1.5, which isn't even close to 50. What the hell do they teach kids in school these days?

Re:3 people in 2 don't know math.

[twistedsymphony]

Except that the American government already tells you the consumption as well as the MPG for every car sold in America. Go visit FuelEconomy.gov

They tell you not just the MPG, but

• the average cost per mile
• the average annual fuel cost
• the average cost to fill up
• tank size
• mileage range on a single tank
• average consumption per year in barrels
• Annual Tons of CO2 Emitted
• .... other crap you might care about

it even has calculators so you can plug in your own average miles traveled and driving style and determine how that effects the above.

also on there has a "garage" feature where you can have your cars and plug in how much you paid for gas and how many miles you got per gallon. (you don't even have to do the math yourself) and it will plot your mileage and allow you to compare what you're getting against the car's listed ratings as well as other drivers with the same car as you. I've got complete gas mileage charts for ever car I've owned over the last few years thanks to that site.

The site was created by the American government, it's been around for years, and it's openly available to any American smart enough to conjure the phrase "Fuel Economy" and plug it into Google.

Not everything needs to be stickered to the damn car... I'm sure you friendly neighborhood car salesman would be happy to give you the sheet that explains all this information about the car too. Don't assume that the only information available is only what's fed to you in TV ads...

Re:3 people in 2 don't know math.

[Opportunist]

Duh, after I have had seven fifths I have trouble with math as well.

Re:3 people in 2 don't know math.

[Anonymous Coward]

> Still I think it's easier to compare lp100km rather than mpg. It's trivial to calculate how much a kilometer costs me. Not so with mpg.

You're kidding, right? It's a simple reciprocal.

20 MPG = 1/20 GPM = 5 gallons/100 miles.

It doesn't matter whether it's gallons/liters or miles/km, the relationship is equivalent. That's why this article is red herring.

Re:3 people in 2 don't know math.

[mbeware]

But when I bought my last car, I looked squarely at the L/100km measurement. I did the math. And I realized that for what I'd save in the ticket price between buying a 2007 Chev. Aveo (which I did end up buying) over buying something like a Smart car or a hybrid, I could pay for the total gas I'd use over the entire lifetime of the car. The difference between 6L/100km and 3.8L/100km just isn't *that* significant, and certainly isn't significant enough to justify spending another $15,000 on a car that probably won't see 100,000km in its entire 5 year lifetime before I trade it in for a new one. The price of gas would have to be 7x what it currently is for that to have become an economical proposition.

The choice for an Hybrid or a fuel efficient car should not only take in to account the cost of the fuel, but also the cost to the environment. If you add the cost of a "carbon fee" for the extra fuel you burn, for the energy required to bring that fuel to you, the damage to the environment to extract that fuel, you get quite a good deal...

The question is still absurd...

[alexandre]

I get that the 1st one is a 100% increase while the other is only 50% but you still get a better deal and less pollution by buying the 50 mpg car (if the price is the same).

So which saves more gasoline? the 2nd one ...

Re:The question is still absurd...

[SleazyRidr]

There was one comment on TFA which pretty much summed it up for me. Imagine a 2 car family. They have a small car that gets 33mpg for zipping around the city, and a big car that gets 10mpg for more serious work. Would they be better off upgrading the 33mpg car for a 50mpg car, or upgrading the 10mpg car for a 20mpg car. if they upgrade the small car they'll save 1 gallon for every hundred miles it drives, but if they upgrade the big car they'll save 5 gallons for every hundred miles they drive.

Of course the answer depends on how much each of them is used, upgrade price etc. but the fuel usage is an important piece of information.

Re:The question is still absurd...

[betterunixthanunix]

It really depends on the comparison you are making. If you currently have a 10MPG vehicle, and you have a choice between 20MPG and 50MPG, clearly the 50MPG car is the winner. On the other hand, if you have two families, one driving a 33MPG compact, the other driving an old 10MPG station wagon, and you can choose to encourage the first to buy a 50MPG hybrid or encourage the second to buy a 20MPG SUV, which policy should you pursue?

The point TFA is trying to make is that there are a lot of people out there who need a large vehicle, perhaps because they have a large family, or because they need to transport some sort of equipment around for their work, or whatever. Such vehicles are not going to get 50MPG (at least not with the current state of car manufacturing), but 20MPG is not unreasonable.

Re:The question is still absurd...

[Anonymous Coward]

I get that the 1st one is a 100% increase while the other is only 50% but you still get a better deal and less pollution by buying the 50 mpg car (if the price is the same).

So which saves more gasoline? the 2nd one ...

I think the point they are making is that a 100 mile trip takes:

10 gallons for a 10 mpg car
5 gallons for a 20 mpg car (saves 5 gallons over the 10 mpg car)
3 gallons for a 33 mpg car
2 gallons for a 50 mpg car. (saves 1 gallon over the 33 mpg car)

    So, creating incentives to get rid of the lowest mpg car saves the most fuel (five times as much).....as opposed to creating incentives to squeeze more mpg out of already efficient cars.

    That is their point. However, in context, if you create incentives to build 20 mpg cars...you actually create a DISINCENTIVE for people to adopt ultra high 33 or 50) mpg cars....effectively reducing the overall fuel savings.

    Changing units will not help Americans understand math...especially when those units are based on 100% gasoline, and not this Ethanol filled crap they sell us at the pump, which DESTROYS gas mileage.

    That, to me, is the biggest "misunderstanding" in the USA today: The addition of Ethanol significantly destroys fuel mileage, destroys engines and components (requiring new parts constructed of Petroleum), AND it makes the gasoline MORE EXPENSIVE THAN IT WOULD BE WITHOUT THE ETHANOL ADDED (once you factor in all the tax money that is given to Ethanol producers...coming right out of our pockets). In short, ETHANOL DOES NOTHING TO REDUCE OUR PETROLEUM CONSUMPTION OR COSTS....it is simply a giveaway to the powerful corn lobby.....

    Also? The use of Ethanol in gasoline drives up the local and worldwide prices for meat (fed with corn), corn, sugar, milk, and a host of other agricultural products that we eat every day.

    If we want to help reduce our Petroleum consumption, step one is to BAN THE USE OF ETHANOL in GASOLINE.

Re:The question is still absurd...

[SEAL]

Addition of 10% ethanol, which is common, does not have a significant impact on gas mileage and it certainly does not "destroy" it. A quick look at Wikipedia on energy density shows that a 90-10 gasoline-ethanol mix produces about 93% of the energy of pure gasoline. Now if you're talking about "flex-fuel" E85, then yeah, you're going to see a noticeable mileage drop. But that is not a common fuel.

Also ethanol acts as an oxygenate, to make the gas burn more completely and reduce carbon monoxide emissions. Pricewise - prior to oxygenating with ethanol, we were using MTBE anyhow, which has health risks and is a ground water contaminant.

So your post is mostly over-hyped nonsense. Now in a general sense, should corn be used to produce ethanol? No, and that's a result of the lobby that you mentioned. But they've affected much more than ethanol usage (see: high-fructose corn syrup and how they essentially killed sugarcane production in Hawaii).

Ethanol production from sugarcane, and to a lesser extent, beets, is much more energy efficient than corn.

Re:The question is still absurd...

[dakameleon]

You don't exactly see your local plumber driving his supplies in an Escalade, though. GP's point is about SUVs etc owned by people who don't use that extra "utility", not actual utility vehicles used for actual utility.

Re:The question is still absurd...

[ScentCone]

Perhaps you have no idea what they do when you're not watching them on the particular road you saw them on. Perhaps you don't see them when they're pulling a trailer, or hauling a soccer team around, or carrying three kayaks, etc. I guess you'd rather that person have a second vehicle manufactured for him, which he can separately maintain, insure, and store somewhere. That's super efficient.

Re:The question is still absurd...

[electrosoccertux]

90-10 gasoline-ethanol mix produces about 93% of the energy of pure gasoline

In other words, the last 10% only gives you 3% more power?

Breaking! mlpm

[drDugan]

Breaking: In an astounding fit of partial international cooperation and scientific rationality, the US adopts a mostly metric measure of resource use: the milliliter per mile, or the mlpm

For example:
10MPG = 378 mlpm
20MPG = 189 mlpm
33MPG = 115 mlpm
50MPG = 76 mlpm
90MPG = 42 mlpm

The unit is linear, easy to understand, with numbers everyone can grasp (40-400 ish), and most important, it slowly creeps the US mind toward the metric system, one small step at a time! What a breakthrough! When the cars fly, we can try for using km, not miles.

Also, mlpm helps put the idea that gasoline is a great resource, to be used sparingly, by the milliliter, as opposed to "by the gallon" like 7eleven slurpies.

Sadly, in all seriousness, from TFA "Consumption instead of mileage? Nah. Dumb idea. Never work. [sigh]" Probably have to agree with this. Not because it's a dumb idea, but because Americans with the social and business systems in place have shown repeatedly that they will hold onto current ideas so strongly even in the face of overwhelming and obvious evidence showing them to be wrong. Only the real American idol will effect real change in the US system, the dollar.

GP100M

[ceeam]

I always thought that measuring it Euro-way - in, for example, gallons per 100 miles - would me more practical and clear.

MPG and GPM are both useful

[JesseL]

Though it may not be obvious why to someone in a metropolitan area or Europe.

MPG is the more useful number when you need to figure out what the range of a vehicle is (and perhaps if you'll be able to reach the next station). In the western US it's not unheard of to find yourself 100 miles from any gas station.

Re:MPG and GPM are both useful

[JesseL]

High octane equals higher resistance to uncontrolled preignition (aka knock, or detonation). Higher resistance to preignition allows more advanced ignition timing. More advanced ignition equals higher cylinder pressures and longer burn duration. Higher cylinder pressure and longer burn duration allows more complete combustion and higher efficiency.

Modern vehicles with knock sensors can get greater efficiency from higher octane fuels due to their ability to keep ignition timing as advanced as possible without running into preignition.

Re:MPG and GPM are both useful

[JesseL]

It's not like that? It's exactly like that. Your car has made you into an idiot.

Re:MPG and GPM are both useful

[ThosLives]

Two words: spark timing.

Many words: Higher octane gasolines are less likely to detonate (knock) at a given air charge temperature than a lower octane fuel. For high compression-ratio engines (those that say "93 Octane recommended" for instance), using a lower octane fuel will retard the spark timing (the spark will be later in the cycle) and reduce fuel economy, because energy available per combustion event is greater for earlier sparks (up to a point). However, if you have a car for which 87 octane is recommended, increasing octane won't really benefit you because the spark timing is already relatively late. Put another way: if you have a high compression engine, lower octane will retard your spark timing and reduce your fuel economy. If you have a low-compression engine, increasing octane won't improve your fuel economy because the spark timing will not increase.

Re:Solution?

[Tuidjy]

In Europe, or at least every country I've lived in, people measure fuel consumption in Liters-Needed-For-100-kilometers. I think that it works better than the way we are doing it here in the US.

Re:Solution?

[selven]

Yes, changing the units will most definitely help. The units we should change to are the ones we already use here above the border: liters per 100 km. Going from 20 L/100k to 15 L/100k saves just as much as going from 10 L/100k to 5 L/100k. In most people's lives, the distance you need to travel is constant, not the amount of money you have to spend on fuel, so fuel per distance is much more logical anyway.

Err..actually its the second one

[101010_or_0x2A]

If you go from 10 to 20 mpg, youre still less than the 33 mpg lower limit in the second case, so the second option "saves more gas". If the question is "which is a bigger improvement in fuel economy", then the answer is the first one.

Re:Err..actually its the second one

[bledri]

Unfortunately, the slashdot summary of the blog paraphrasing of the Duke study failed to spell out the real question which is: Given a two car family that have (and presumably needs) both an SUV and a commuter and both are driven 100 miles per week. Does it save more fuel to replace the 10 mpg SUV with a 20 mpg SUV, or the 33 mpg commuter with a 50 mpg commuter. Most people pick the wrong combination. Here's an example online test: MPG Illusion

Re:Solution?

[Anonymous Coward]

True, but you're one of those people who want to use the metric system and hate America, so I won't listen to what you say.

Forget mpg.

[gklinger]

My car gets 40 rods to the hogshead and that's the way I likes it.

Re:Solution?

[CecilPL]

My Accord gets about 10L/100km, or 0.01m^3/100000m or 1e-7 m^2. This is equivalent to one tenth of a square millimetre.

Which means a strip of gasoline down the centre of the lane, with a cross-sectional area of one tenth of a square millimetre, would be enough to power my car as it drives.

Re:Solution?

[Ken_g6]

So let's use English units (even though England doesn't even use them anymore!)

There are 128 fluid ounces (oz) in a gallon. So oz/mile (let's call it OPM) is equivalent to gallons/128 miles.

This also happens to be about half the European value. (5 L/100km == 2.72 OPM == 47 MPG)

I have used this site to track my last two cars

[Shivetya]

and will be using it to track my latest.

The first car I tracked was a 2007 Civic Coupe EX, manual transmission. The second was a 2008 Mazda Miata SE. Without bouncing to the site to check the Miata was rated at 21c and 28h. In my two years of recorded mileage I never fell below 27 in a mix of city and highway travel; I have a 26 mile commute mixed between 35 and 55 with no interstate. I peaked at 33 during summer months and fell into the 27 range during winter where besides the cold requiring the engine to run rich I needed the compressor to run the defroster.

My newest addition, a 2010 VW Golf TDI with manual transmission shows that others have never been as low as the EPA predicted mileage which is 30c and 41/42h. I have yet to finish my first tank and the car's computer shows 38 miles per gallon.

Why all the fuss? Well the facility is there and it gives me a nice reference to see how my car reacts to seasons and how it reacts to age to include over all miles traveled. It provides a nice little log so I can also see fluctuations in fuel prices without having to rely just on my memory; like what was the price of gas in June 2008.

While many cars are not represented it does appear those with cars known to get good mileage do post their numbers more often. I have been surprised while browsing some vehicles to see just how low the mileage ratings are for many popular sedans, in many cases not better than many SUV/CUV types that people love to vilify.

In the rest of the world

[AliasMarlowe]

Outside the US & UK, fuel consumption for cars, trucks, etc, is normally expressed as Liters per 100 kilometers. This completely avoids the non-issue that TFA is grumbling about, even for innumerate consumers, since the numbers represent fuel used in a trip of 100km.
10 miles per US gallon = 23.52 L/100km (mind-bogglingly bad)
20 miles per US gallon = 11.76 L/100km (very bad)
33 miles per US gallon = 7.13 L/100km (OK for SUV, not so good for a car)
50 miles per US gallon = 4.7 L/100km (good for medium or large car, not so good for compact car)

FWIW, my Mercedes diesel stationwagon uses about 5.5 L/100km for mixed city/rural driving, which is 42.8 mpg(US) or 51.4 mpg(UK).