The sources of fuel efficiency: a counterintuitive result

by on December 23, 2007 at 6:37 pm in Data Source | Permalink

Matt Yglesias writes:

Via Andrew Sullivan, Eric dePlace notes that "You save more fuel switching from a 15 to 18 mpg car than switching from a 50 to 100 mpg car." And so you do. A 15 MPG car would require 1,000 gallons of gas to drive 15,000 miles while an 18MPG car could get it done in just 833 gallons. That saves 167 gallons of gasoline. By contrast, since a 50 MPG only uses 300 gallons to go 15,000 miles, upgrading to 100 MPG can’t save that much gas — the super-efficient car uses 150 gallons.

It is tricky, because the consumption basket and number of miles driven will not stay constant across alternatives, but this logic is worth keeping in mind nonetheless.

Isabel December 23, 2007 at 6:52 pm

In some European countries fuel efficiency is measured in liters per 100 km. Say we did analogously and measured fuel efficiency in gallons per 100 miles. Then the 15, 18, 50, and 100 mpg cars require 6.67, 5.56, 2.00, and 1.00 gallons per 100 miles and the results make perfect sense.

In short, one needs to measure things in the “right” units.

Daniel Yokomizo December 23, 2007 at 7:21 pm

It doesn’t matter if we measure them in mpg or gpm, the result is “counterintuitive” (scary quotes required) because the percentual increase on efficiency can only give so much benefits. As your efficiency increases it’s harder to get more savings, because you have much smaller expenses in the first place: 16.7% of 1000 is higher than 50% of 300. It’s obvious if we track expenses instead of efficiency. Using the gpm measure the savings are the same: 1.11 (i.e. 6.67-5.56) is higher than 1.00 (i.e. 2.00-1.00) so the total saving from the first transition is still better than the second transition.

This kind of reasoning is useless anyway. What matters is the ROI of each choice. If the savings from both transitions offer good ROI why bother on how much are you saving (assuming there’s no cost of opportunity)?

John December 23, 2007 at 7:47 pm

This is a pointless argument because all for vehicles are not compared side by side and give the illusion of counterintuitiveness.
15mpg car 1000gals/15000 miles
18mpg car 833gals/15000 miles
50mpg car 300gals/15000 miles
100mpgcar 150gals/15000 miles

of course there is a law of diminishing returns, but the counterintuitive argument would lend some less educated people to think that in order to help the environment and my wallet I should choose an 18mpg car over a 100 mpg car because it “saves more fuel” which is patently ridiculous. Assuming all four cars looked the same and performed at relatively the same speed, acceleration, braking and comfort, which car do you want?

Alan Coffey December 23, 2007 at 8:27 pm

Some of you seem to be so educated that you miss the illusion that a less educated person might operate under. That is a good thing.

Increase my mileage by 3 mpg and I save 167 gallons.
Increase my mileage by 50 mpg and I save 150 gallons.
WHAT!?!

Well, you have to know the scale over which you make the change of course.

Does this have policy implications? Should we institute a minimum mpg standard instead of, or in addition to, the CAFE standard as calculated for fleet economy? Since relatively small improvements in mileage generate large returns at the bottom of the scale and relatively less at the top?

A student of economics December 23, 2007 at 8:56 pm

I think Isabel’s and Tomr’s comments above and the Europeans are on the right track. Instead of MPG, cars should report GPM or gallons per 10,000 miles. This would make it clearer to the suburban family that its much better to trade in the old gas guzzler for a more efficient minivan than to replace a fairly efficient compact with a hyper efficient one.

More broadly, if you want to reduce oil use (and pollution), the focus should be on getting the gas guzzling cars and trucks off the road. The push by some politicians to have lower efficiency standards for “trucks” like hummers and SUVs, etc. than the the rest of car fleet can be seen to be especially foolish in this light.

Dano December 23, 2007 at 10:17 pm

Would people drive the same amount of miles or would they drive more miles in super efficient cars?

John December 23, 2007 at 11:43 pm

More broadly, if you want to reduce oil use (and pollution), the focus should be on getting the gas guzzling cars and trucks off the road. The push by some politicians to have lower efficiency standards for “trucks” like hummers and SUVs, etc. than the the rest of car fleet can be seen to be especially foolish in this light.

This is true– but at the same time is a caution against raising standards “too fast,” if it results in people keeping older cars longer rather than buying new ones, as older cars get worse mileage just from wear. (Also, older cars have much, much worse emissions in other ways, because of the slow improvement in standards over the years.)

Another approach is to reduce the total miles of commute, which is why Mr. Yglesias has noted before that changing zoning to allow greater densities in city centers would help as well. DC is particularly bad in this regard.

A student of economics December 23, 2007 at 11:55 pm

“his is true– but at the same time is a caution against raising standards “too fast,” if it results in people keeping older cars longer rather than buying new ones”

Agreed. The more direct, and effective, policy is to simply tax the products with the negative externalities, e.g. pollutants, or near proxies, e.g. gasoline and other fossil fuels and let people buy whatever cars they want (as long as they pay for the externalities they create). Use the revenue to replace some of the taxes on activities with zero or positive externalities (labor, capital, research, etc) and its an obvious welfare gain.

8 December 24, 2007 at 10:23 am

I thought this was one partial explanation for why SUVs became so popular during the late 1990s and early 2000s.

Javier December 25, 2007 at 1:36 am

The “counterituintive result” is driven by a comparing MPGs (as a ratio), with savings in absolute gallons consumed. If you keep all units as percentages, logs or ratios, there is no puzzle at all:
Going from a 15 to 18 MPG car saves you 16.6% (3/18=0.166) on gas because GPM efficiency increases in 16.6% (167/1000 = 0.167)… And going from 50 to 100 mpg saves you 50% on gas because GPM is also reduced in half(150/300=.5)
Duh?

Jarick December 26, 2007 at 12:05 pm

You would have to force people to change their preferences in order to change their lifestyles. I wouldn’t live in my downtown “city center” if rent were completely free.

I agree about the diminishing marginal effeciency. What the efficiency remark says to me is that the biggest potential savings in fuel consumption is in the SUV and truck area. Moving someone from a V6 Accord (24 mpg) to Prius (45 mpg) nets an annual savings of 292 gallons. Meanwhile moving from a Durango (12 mpg) to a V6 Accord (24 mpg) saves 625 gallons.

So we see the rise in popularity of more fuel efficient vehicles. People are moving from Civics to Priuses, from Accords to Minis, from full-size SUV’s to crossovers. I really don’t see the need for government regulations, unless it includes a massive subsidy for the American auto manufacturers who are getting killed because they hedged all their bets on the longevity of SUV and truck sales.

odograph December 26, 2007 at 5:08 pm

I think I talked someone into jumping from a Ford Expedition to a Prius this last week (14 mpg -> 46 mpg). It’s not impossible.

Heck, I used to own things like Jeep Cherokees. I use a Prius for the same things, probably because like a lot of SUV owners I didn’t really need it. I just kept waiting for that flood, or for the tree across the road.

(One time the road did flood, but the police waved me around, wouldn’t let me ford it the Jeep anyway.)

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