Greenlings: How does weight affect a vehicle's efficiency?
by Sebastian Blanco on Oct 29th 2009 at 8:06PM 
Over the last few decades, the average weight of a vehicle sold in the U.S. climbed steadily after we got over the oil embargoes of the 1970s. Today, though, auto companies are putting a lot of effort into reducing weight – Lotus set up an entire lightweight structures division, BMW is investing millions into carbon fiber and Jaguar loves aluminum – because every ounce you take out of a car improves the vehicle's performance and fuel economy. Options for weight savings that automakers are investigating include installing things like plastic fuel tanks (PDF) and using carbon fiber instead of steel. As we discovered in a previous Greenlings, carbon fiber is a remarkable, lightweight substance that will likely not be used widely until prices come way, way down. Today, one of the main reasons automakers want to reduce weight is because it's a great way to increase MPG numbers. AutoblogGreen reader GenWaylaid sent in a Greenlings question about how, exactly, reducing weight helps efficiency. We investigate his query after the jump.
Let's start with the easy and simple numbers. The EPA says that for every 100 pounds taken out of the vehicle, the fuel economy is increased by 1-2 percent. Based on a gallon of gasoline costing $2.58, this translates to savings of between $0.03-$0.05 a gallon. Of course, 100 lbs. in a small hatchback is going to make a bigger difference than those same 100 lbs. in a Tahoe, so make reasonable assumptions about what going lightweight can offer you.
For a more detailed look at what's possible, we turn to a report issued by the Aluminum Association, Inc. based on research by Ricardo. The chart below show that for a small car with a 1.6-liter engine, reducing weight by five percent led to an increase in fuel economy of 2.1 percent on the EPA combined rating. Eliminating 10 percent of the weight gave a 4.1 percent mileage boost and a dramatic 20 percent weight decrease improved fuel economy by 8.4 percent. To find out how other vehicle types fared, download the PDF. (See page 35 of the report for a great chart showing the effect of 100-lb. reductions on different vehicle types in different situations).
To download the full PDF report, click here.
Ways to remove weight in your own car
Since the automakers have already made their decisions regarding how heavy your vehicle is going to be, it's up to each driver to eliminate weight whenever and wherever possible. The place to start is in the trunk and in the back seat. Got some old boxes in there you never use? Put 'em in the garage. Been carrying around a set of golf clubs since last weekend? Put 'em aside for now. Got a dead body in there? Um, that's an entirely different set of problems.
Once the obvious detritus is removed, there are a few other ways to lighten the load. While we have to admit that losing a bit of belly fat can technically make a difference and is probably a healthy choice, we don't want to put too much emphasis on that angle – it's been called out already. People who know what they're doing (and by this, we mean they have a reliable back-up plan, either a cell phone and time to wait or AAA or something) sometimes ditch the spare tire and just deal with it when a tire goes flat.
An extreme example of a way to driver around with less weight would be to only fill up the tank half way. Sure, you're trading time for efficiency, but if you live near a gas station and don't drive too often, this could be a reasonable thing to consider. Gasoline weighs about 6 pounds per gallon and diesel about 7, after all. Filling up to just half of a ten-gallon gasoline tank means you're taking 30 lbs. out of the car. Consider it.
The weight of the future
One of the rarely discussed realities of the U.S. auto industry is that even as fuel economy ratings for most vehicle classes stayed about the same for the past few decades, the vehicle themselves have gotten heavier with all of the added entertainment, comfort and safety features. These numbers were able to diverge like this because engineers were making the vehicles more efficient in ways that didn't involve saving weight. Now that the industry is focusing on shedding pounds – something that will become even more important once heavy automotive batteries for plug-in vehicles start appearing more and more often – the gains made with heavy cars can be applied to lighter vehicle. After all, U.S. cars still have a long way to go to reduce weight, and we'll all reap the benefits thanks to reduced fuel usage.
Reader Comments (Page 1 of 2)
Rain 7:28AM (10/30/2009)
Run Synthetic oil in all the sumps.
Check the tires for proper inflation.
Preform regular maintenance as scheduled.
Check tire alignment and suspension components for proper operation.
Drive smoothly and avoid driving under emotional duress.
Do all errands at one time and walk or ride a bike for those impulse errands.
Buy stuff made and grown locally and You are supporting You friends and Neighbors
and keeping Your money away from terrorist's,socialist's and the corporate asshats.
Being green never felt so good or healthy.
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Laurens 8:58AM (10/30/2009)
How to reap the benefits of green motoring, if you're not motoring at all ;-) It does make a lot of sense, your approach. Pity the marketing fluf seems to push us in a different direction. EV crisis next?
Jared 12:47PM (10/30/2009)
Buying locally does more then prevent all that political B.S., it helps the environment and your body as well. When I moved to Florida from South Carolina, I had REAL bad allergies. One of my friends suggested the old wives tale of eating only local honey, and eating atleast a Tablespoon a day for the first week. It's said that since the honey is made from the local flowers, and thus has local pollen in it, it helps your body get used to the surroundings faster then doing nothing. Along with buying that local honey, buy the rest of your fresh products and cut down on gas used by shipping companies.
Nateb123 5:41PM (10/30/2009)
Well Ma'am, it seems like you've got a pretty serious hippy infestation here. A full-blown drum circle in the yard is no laughing matter...
Seriously, come on. This is a topic on engineering and you hippies are talking about buying local produce? Stop spewing your own anti-corporate, pseudo-naturalistic dogma to anyone who will listen. It's obvious you all just need a little pat on the head from each other because you know deep down you're hugely pretentious losers.
Plus if you were going to comment on driving economically properly, you would have included using engine braking. Stupid hippies.
Andy 3:30AM (11/01/2009)
Awwwh ...... Nateb 123 you are so mean to hippies.
Looks like you need a little love and some space to grow up.
Nozferat 3:46PM (11/07/2009)
NATEB:
I wonder who's the bigger pretentious douche actually...a hippie who actually gives a crap about their surroundings and doesn't their lives run by corporations or someone like you who believes that corporations should keep going with their waste, rampant pillaging, and the destruction of the same environment you so undeservedly use everyday to keep your pathetic self alive.
Hmmm?? Which do you think is more pretentious?
Nozferat 3:49PM (11/07/2009)
RAIN:
You're blaming socialists and terrorists for your greedy self-indulgent lifestyle? Hmmmm...I wonder how that works.
Rain 5:15PM (11/07/2009)
Nozferat,
You imply that I am blaming all Socialists and a few religious zealots for My own lifestyle.
My lifestyle is My responsibility and I tend to it daily by working to buy things for survival and to help others.
I could live a more opulent life but I choose not to.
I've been there and I know that it doesn't bring happiness,so now I serve others in My community.
In My role as a Civil Servant,I make roughly one third of the income that is available to those in the private sector.
I have no grudge against those People,much like Myself,Who are struggling to survive,.
I do have issue with People who feel it is Their right to dictate the terms of that survival for profit and Their own em-betterment.
I am sorry,but Socialists are property of the state and I fear that given half of a chance the People at the top here in America would love to put themselves in that position of Ownership
over the rest of the World.(see Manifest Destiny)
The way to get rid of a pest is to make it get its food elsewhere and what I propose is simple:
Reduce,Re-use,Recycle and live within Your means.
What this World does not need is another fatwa or intifada,what it does need is a level playing field for the People when it comes to acquiring the basics for life and then perhaps there won't be such a need for religion or greed or power,having first defeated thirst and hunger and fear and ignorance.
No form of Government has successfully addressed these issues,so it falls to each of Us.
Nozferat 12:48AM (11/09/2009)
You seem to have a very self-centered, self-revolving, egocentric view on things...perhaps that's your problem. What you do affects everyone else....however little it may seem to you. So your lifestyle IS other peoples' problems somewhere down the road.
Obviously DaMinority 11:43PM (10/30/2009)
Seriously? You have to ask? I have a simple answer.
One word:
Physics.
Take it, learn it, love it and hate at the same time.
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Mark Kiernan 8:51AM (10/30/2009)
terrorist's,socialist's (you are using the possessive not the plural) ;-)
but I agree with your statement (not sure what socialists have to do with this though), I wish some Americans would educate themselves as to what a socialist is, and you will see that Obama is anything but a socialist.
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mike 2:45PM (10/30/2009)
Obama is a crony capitalist, not a socialist.
Lets see, a government that:
Set pay for company's executives - check
Owns stock in GM, Chrysler, AIG, Citigroup, etc. - check
Nationalize healthcare - check
Demonizes the Chamber of Commerce - check
Floats proposal to 'bailout' newspapers - check
Bails out Fannie mae, Freddie Mac with treasury funds - check
Rain 5:39PM (10/31/2009)
Mark,yep,My English is patchy at best.
It should have read;Terrorists and Socialists and corporate Asshats.
Sorry for the err and thanks for the comment,You too,Mike.
Except,by Socialist,I meant Socialists in South America and the former Soviet Socialist
Republic where a large part of Our imported oil comes from.
Now on to the fashionable term of Socialist as in circa 1902 AD made popular by elements of the GOP while trying to stir up animosity towards the Democratic party and also circa 1950 known as the Red Grange and the Joseph McCarthy's Un-American Activities investigation,respectively.
Here is Webster's definition of Socialism,read it,own it:
Pronunciation: \ˈsō-shə-ˌli-zəm\
Function: noun
Date: 1837
1 : any of various economic and political theories advocating collective or governmental ownership and administration of the means of production and distribution of goods
2 a : a system of society or group living in which there is no private property b : a system or condition of society in which the means of production are owned and controlled by the state
3 : a stage of society in Marxist theory transitional between capitalism and communism and distinguished by unequal distribution of goods and pay according to work done
So in a socialist society,the State owns everything.
We are more into a Corporate-Feudalist society where the 1% of the population
owns the remaining 99% such as existed 150 years ago,google the Robber Barons and Neo-Con paradise for comparison of past and present terms and definitions.
If the State owns it then You may not own property,thus You are property.
Mr.Obama simply is not that sort of fellow,I promise You that.
He is trying fix the problems left behind by another political faction which I will not stoop
to labeling here other than as being mis-guided Americans.
As for the hippie comments from some of the others,well;to quote some People from My generation;'Up Yours'!
MikeW 6:32PM (10/31/2009)
Obama works for wallstreet.
He is an anti-socialist.
He is 'fixing' nothing.
Has his impeachment started yet?
ChoadNamath 9:28AM (10/30/2009)
"Based on a gallon of gasoline costing $2.58, this translates to savings of between $0.03-$0.05 a gallon."
You might want to double-check your math there. Losing weight doesn't make gas less expensive, it lets you drive a longer distance with a given amount of gas.
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Doctor_D 9:50AM (10/30/2009)
Well only running with a half of a tank of gas may make sense from a weight perspective, but my car--for whatever reason--typically pulls 250-300 miles on the first half tank, and then maybe a 100 on the last half tank.
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lne937s 10:12AM (10/30/2009)
you are filling your gas tank beyond what the meter is reading as full. Full is measured as near the top of the tank, but additional fuel fills the top of the tank and the tube leading to the tank- beyond what your fuel gauge measures. There also tends to be a little margin of error built in at the bottom of the tank so that people do not run out of gas... so unless you run it to full empty, you really do not know what is going on there either.
Despite what your gas meter is reading, you do not get better fuel efficiency with a full tank.
protomech 12:29PM (10/30/2009)
Doctor_D: Fill the tank by volume and not by the indicated halfway point. Eg if you typically fill 14 gallons from near empty, fill 7 gallons when you hit your usual near empty point.
However, filling half a tank is a good example of premature optimization. Your fuel economy might improve by 0.2 - 0.5%. That fuel economy increase will quickly be destroyed when you have to make an additional trip to the gas station per full-tank. Even if you stop and fill up while out running errands, you will burn enough fuel to erase your fuel savings. 0.2 - 0.5% would give you an extra 0.8 - 2 miles on a 400 mile tank, less considering the maneuvering and startup costs of making an extra pass through a filling station.
LloydChiro 1:24PM (10/31/2009)
Doctor_D, My car does that too! I figure if i fill my car up always at the 1/2 tank line, I'll probably double or triple my mileage. Works every time.
KK 11:11AM (10/30/2009)
This article is ridiculous.
The numbers in the first paragraph seem reasonable. But reducing the weight of a car by 10% is completely unrealistic - that's 250 lb reduction for a 2500 lb car. It's pretty unlikely that you have even 60 lb of unnecessary weight; removing 60 lb would improve the fuel economy by less than 1%.
The correct conclusion is: "Carrying a bit of extra weight does NOT noticeably reduce the fuel efficiency. It's just silly to follow that with tips for removing extra weight from a car, like filling the tank halfway (maybe 30 lb weight reduction). There are FAR easier ways to save gas, like combining multiple trips into one, reducing highway speed by a few mph, etc.
And for what it's worth, I used to have a Dodge minivan; this vehicle DID have a lot of removable weight, namely the middle and rear seats. I couldn't detect ANY change with the "divide the trip odometer reading by the amount of gas I just bought to fill it up" method.
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