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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."
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.
No good deed goes unpunished...
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.
Is 1563649 a prime number?
You fail, hard. The first scenario goes from 0.1 gallons per mile to 0.05 gallons per mile. You're saving 0.05 gallons of gas with that switch. The second scenario goes from 0.03 gallons per mile to 0.02 gallons per mile... or only 0.01 being save. Making the first switch saves 5x as much gas as the second.
If raw numbers are too hard to understand, imagine going from a two-liter of gas per mile to a coke-can of gas per mile, vs. going from a thimble of gas per mile to half a thimble of gas per mile.
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.
"Prefiero morir de pie que vivir siempre arrodillado!"
That particular asylum is run by the inmates (That is, Energy Star ratings are paid for by the manufacturers and there is little oversight).
Nerd rage is the funniest rage.
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.
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.
"There are a dozen opinions on a matter until you know the truth. Then there is only one." - CS Lewis (paraprhase)
They tell you not just the MPG, but
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...
Collector's Edition
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.
* Winners compare their achievements to their goals, losers compare theirs to that of others.
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
Some privacy policy Slashdot.
See... in Canada, most large appliances have exactly that measurement plastered on a sticker on the door. kWh/year estimated. And for the most part, these are the same brand names (and probably the same appliances) you can find in Europe and the US... brands like LG, Whirlpool, and Maytag.
So the testing is already done. And admittedly, it's very difficult to accurately measure the overall usage for an electric stove (one of the main reasons I use a gas stove, though they're generally more efficient anyway, as they take less time to heat up/cool off, so less energy wasted), but for something like a fridge or an air conditioner, it's a better measure. I think they even have a "standard" duty cycle that they include in the measurement specifically because something like an electric stove or microwave won't be on 24/7.
And on topic, we already use L/100km for measuring cars' efficiency, too. You'll find it right next to the old MPG measurement for people who are used to that. 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.
> 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.
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).
Those who can make you believe absurdities can make you commit atrocities. - Voltaire
I mean, you "get" a certain amount of miles out of a gallon of gas instead of "needing" a certain amount of gas to go 100km.
America's a pretty large country and we tend to take a lot of long-distance road trips. When I used to visit the in-laws, we'd pile into the family minivan and drive 1,100 miles each way to get there. As others have said, MPG is good for calculating range. If I filled up with gas in Chicago, should I get more gas in Cleveland just to be "safe", or can I wait until I'm in Pennsylvania?
Still I think it's easier to compare lp100km rather than mpg. It's trivial to calculate how much a kilometer costs me.
For my family, the cost of fuel is unimportant. For daily driving, it adds up slowly enough that other expenses like maintenance and insurance are more important considerations. For long trips, the cost can be significant, but not enough to deter the trip even if we had to pay twice as much. Now, fuel economy will still factor into my next purchase, but once I own the vehicle, it's just not something that I care to think about. I already have the thing and I'll pay to put gas into it whenever I need to. Once I've done that, I'm more interested in how far I can go until I have to do it again.
Dewey, what part of this looks like authorities should be involved?