Fuel Efficiency for Dummies
Which would save you more gas? Trading in your car that gets 25 mpg for one that gets 50 mpg? Or switching from a car that gets 10 mpg to one that gets 20 mpg?
The answer is not what you'd expect. Actually, switching from 10 to 20 mpg is a bigger savings...
Look at it this way. If you drive 100 miles in the 50 mpg car it takes only 2 gallons. But in your current car getting 25 mpg it takes 4 gallons. So switching cars would save you only 2 gallons over 100 miles driven.
But the car that gets 20 mpg would burn 5 gallons versus the 10 mpg car that burns 10 gallons. There, the savings is 5 gallons.
This "math illusion," as one expert described it, prevents car buyers from seeing the real picture when it comes to fuel efficiency. So when we go car shopping, and compare the miles per gallon ratings, it hides the true nature of saving gas.
What's the alternative? I'm glad you asked. Here at Edmunds a move is afoot to reframe the picture and present gallons per miles instead of miles per gallon. In other words, how much gas is burned per 100 miles traveled? Per 1,000 miles traveled? And if you look at these figures, it would be easy to see what your yearly fuel bill would be with different cars. Most Americans drive about 10,000 miles a year.
Yes, yes, I know, they already do this in Europe (using liters and kilometers). So the idea is not completely original. Still, I'm thinking this will be a huge step forward for car buyers.
It took me a while to see the difference between the two systems. But then again, when it comes to math, I'm in Homer Siimpson's league. Maybe that's a good qualification for my job. I'm struggling with the basic concepts just like the couple on the car lot who are squinting at window stickers and wondering if they can afford gas for the car they want to buy.
What about you? Anyone out there think this is an improvement? Cast your vote and tell me your reasons for loving/hating the new system.
- Posted by
- Philip Reed June 25, 2008, 3:12 AM
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- Car Buying
I like your play with numbers.
but at the pump, my % savings in both the 10 to 20mpg, and 25 to 50mpg. would be the same.
Yes in absolute dollars, theres a difference, But I would go from a 20 dollar fill up to a 10 dollar fill up.
plus isn't it 'law' that on the window sticker, the manufacture list and 'average' yearly fuel cost?
Which to makes it even easier to compare costs, since then you have to do no math.
They don't just do it in Europe. Fuel consumtion/economy is measured in l/100km pretty much everywhere outside of the US, including in Canada. Japan is also weird -- they do it in km/l.
We just had this discussion Monday in the Edmunds Green blog:
http://66.160.188.111/.eec0361
Six of one, half a dozen of the other.
This is why the GM hybrid SUV's are actually a big deal. If not for their high cost of admission, the fuel savings over a non-hybrid SUV is pretty considerable, even though the MPG numbers are not that different (20mpg vs 15mpg)
Being math-challenged I'm still wondering: is it easier to see the real picture with this different system. Yes, yearly fuel costs are already on the window sticker. But they are small and hard to remember. So, when you're around the water cooler and one guy says, my car gets 5 gallons per hundred miles, you can laugh and say, "What a gas hog! Mine gets 4."
Or maybe it will just take time before we can change over our frame of reference and have it be second nature.
I agree with Philip. Although gpm and km/l give a better picture as far as comparing cars are concerned, the figures are often ugly and it can be hard to guage how far the car can travel on a gallon without a calculator. So I think the current mpg is better.
I'm thinking the EPA stickers should display bothe mpg and gpm.
Phillip, I'm thinking that rather than "getting" gallons per mile, you are "using" gallons per mile. You only "get" miles per gallon.
At any rate, like opfreak, I am inclined to think in terms of percentages, and the ratio is the same regardless of which is the numerator or the denominator.
Good point ahightower -- with a change in the system we're going to have to change the way we refer to it. "I only use 5 gallons per 100 miles. Your gas hog burns 10 gallons." That will be a big hit with car owners.
Perhaps I'm biased being from Canada but I find the l/100km or (g/100miles) easier to relate to quickly.
e.g. 100 miles per week commuting would cost $20 ($4/gallon) in a 20 mpg vehicle and $40 in a 10 mpg vehicle.
People can tell you pretty quickly how far they drive and g/100 miles is quicker to do the math to figure how much that is costing you. It also emphasizes the degree of fuel inefficiency more clearly. 20 mpg = 5G/100miles vs. 10 mpg = 10G/100miles. Now people are comparing gallons to gallons.
I find I also pay more attention to how long my trips are as I am thinking in a cost per km, while the traditional method probably would have me more focused on the cost of my fill up. It's also easier to show people how little money they save by spending big bucks on a 50 mpg hybrid or diesel versus a 35 mpg compact ~ $4 per 100 miles driven or $400 per year.
Phil's point is this: The value of a 1 mpg change by itself does not tell the shopper much, because the value of 1 mpg in pure gallons or simple dollars varies. Look again at another straight percentage example:
If you drive 1000 miles in a month, the mpg improvement when going from 10 to 11 mpg is 10%. The fuel savings is 9.1 gallons. The 10 mpg car uses 100 g/1000m and the 11 mpg car uses 90.9 g/1000m. At $4.50 per gallon, this 10% mpg improvement saves $40.91 in fuel. And that's just a 1 mpg improvement.
From 30 to 33 mpg, also a 10% improvement, the savings is only 3.0 gallons per 1000 miles. The 30 mpg car uses 33.3 g/1000m and the 33 mpg car uses 30.3 g/1000m. Here a 10% improvement in mpg saves only $13.64 in monthly fuel despite appearances of an apparently "bigger" 3 mpg improvement.
This is because MPG is incorrectly inverted (the dependent variable gallons is in the wrong place). It becomes non-linear: the value of 1 mpg differs depending on where you are on the spectrum. And all of us need gallons to calculate dollars.
With g/mile (or g/100miles or g/1000miles,) the differences are equally significant throughout the range. It's linear: a gallon saved is a gallon saved. And you can figure your costs easily by multiplying by the cost of fuel.
I think the GPM figure is a good one.
they already have the figure for Estimated Annual Fuel Cost right on the bottom of the window stickers. Check to see if you still have yours.
I looked at mine for my 2005 Mustang GT and it was $1350. I don't know how many miles they were using to come up with that figure.
I don't think the gallons/ mile figure would work well in the US. We're all about BIG NUMBERS here!
Using MPG gives us a higher numerical figure to shoot for, which goes great in our competitive culture.
I'm starting to see the power-loving gearheads becoming concerned about mpg (instead of hp and 1/4 miles times) and using the current MPG measure should work well in letting them know about increases in their new "performance" for the current gas crisis.
I remember an article calling for speedometers to show feet/minute which would show higher numbers than the miles/hour figure, but since we never got kilometers/hour here in the US I don't think either will fly!