AskPablo: Lighten the Load

I recently listened to one of my favorite podcasts, On Point with Tom Ashbrook, and heard an interview with Bob Lutz, General Motors’ Vice Chairman. In this interview he expressed dismay that Toyota is thought to be the fuel efficiency leader in the industry. This week I will run some numbers to shed light on the subject.

Fuel economy in the US is regulated on a fleet-wide basis by the CAFE standard (Corporate Average Fuel Economy). To be in compliance with this law the average rating of all cars sold by a company must be 27.5 MPG. Unfortunately SUVs are considered to be “light trucks” and are exempted. While Toyota is known for high MPG vehicles, GM claims to have more fuel efficient vehicles in the sizes that Americans actually want to drive, the SUVs. In fact, Bob Lutz made a claim that GM beats Toyota in terms of fuel economy in most vehicle classes. Could it be that GM actually produces more fuel efficient vehicles than the maker of the Prius? Is it even relevant to look at vehicles by class?
I decided to have a look at 55 cars from various classes, made by Toyota, GM (Hummer, Pontiac, Saturn, Buick), BMW, and Audi. I found their curb weight and their fuel economy ratings and plotted them on a graph. What I found was very interesting… It didn’t really matter which company makes the car, or what class it’s in. The biggest correlation to city fuel economy was vehicle weight. This makes sense because, when a car is being driven around in a city there is a lot of stop and go. From my recent columns on vehicle efficiency (I, II, & III) we know that acceleration uses more energy than maintaining a constant speed. When I repeated my graph using the highway MPG data I found that weight was a bit less correlated, but still very important.
With a little bit of tricky Excel work I found that, for every 100 lbs of weight added to a vehicle, the fuel economy drops by 0.4-1.25 MPG (more fuel economy is lost by adding weight to lighter vehicles). This reminded me of a report that came out a few months ago that claimed that the obesity epidemic in the US is costing us in terms of fuel consumption as well. According to the author we use 1 Billion more gallons of gasoline each year than we did in 1960 just because of our increase weight. In addition to losing a few pounds you can also make sure that your car is not filled with stuff that you could leave at home, like cases of cheap beer, your set of free weights, and that road-kill moose…
Another interesting thing that I learned is that the CAFE standard actually encourages alternative fuel use by weighting vehicles that run on fuels like E85. According to Wikipedia “a 15 mpg dual-fuel E85 capable vehicle would be rated as 40 mpg for CAFE purposes, in spite of the fact that less than 1% of the fuel used in E85 capable vehicles is actually E85.” This probably explains why GM seems to be the only car company promoting ethanol vehicles.
So, when you hear claims of one car company being greener than another look at the size of the vehicles they make. If we want to have a much higher average fuel economy we need to lighten the load. There is only so much that technology can do with a conventional internal combustion engine. Hybrid technology gives us a great efficiency boost but we need to do better. As long as people think that they need a 7-passenger off-road vehicle just because they had a baby or two we are in trouble.
Pablo Päster, MBA
Sustainability Engineer

7 responses

  1. Bravo! I nominate this as one of the best Ask Pablos ever… PLEASE DIGG IT.
    GM also bases their numbers on the claim to have the “most models” that are efficient. Of course, they offer identical models under different brands, so this is basically baloney too.
    Finally, although I have no problem with smaller cars, we could still have larger vehicles if we found a way to improve what they’re made of – ie, the Amory Lovins hpyercar idea!

  2. I’m glad you took the trouble to present to your readers the relationship between vehicle curb weight and gas mileage. Although there’s nothing new about this linkage (I remember it from twenty or more years ago) most Americans never see the connection, probably because the marketers don’t want them to think about it. Lighter vehicles get better gas mileage.
    Just guessing, the data points above the curve are 4 cylinder cars and the ones below are 8s. Eight cylinder engines develop more power will accelerate a vehicle faster but use more gas to do it. Tires, gearing, rolling friction and aerodynamics explain the rest.
    My ’89 Corolla wagon gets 35 city (if I keep my foot out of it) and over 40 on the highway. I’ve guessed that it weighs 2500 pounds.
    Then there’s this tiny 3 wheel car that weighs in at about 750 pounds and would graph in at about 80 mpg. I don’t remember the name.
    Keep playing this song and maybe more Americans will learn the words and sing along. Lighten up, save gas and save money.

  3. Excellent post, but note that the relationship is non-linear. The type of relationship curve represented usually suggests that constant change on the input generates disproportionate change on the output. That’s the case here too.

  4. Great post! There is a recent article examining this relationship, and adding vehicle power into the mix too (here is the GCC summary). As you said, most people don’t use the space afforded by a larger vehicle on a daily (weekly?) basis. It would save the owner money by owning a smaller/lighter vehicle for most uses, and renting a larger vehicle when then needed it for hauling or recreation. Keep up the great work Pablo!

  5. Update: The senate just voted to increase the CAFE standard to 35 MPG by 2020. This new standard now includes SUVs. Car makers are arguing that this standard is not attainable. As you can see in my column it is attainable, if the weight of cars is reduced. Smaller engines, carbon fiber car bodies, and hybrid technologies can all make this happen. Japan’s MPG standard is 45 and Europe’s is over 40. It is about time that the US shows a leadership position on global climate change…

  6. I have a question about your excel graph. You say that it took some tricky excel work, but it seems to me that this is real data. Did you change the weight in your car and measure the fuel efficiency?

  7. Hi Pablo: I am very glad to see your post. Good work. I tried something similar back in June 2005 and decided that a simpler linear relationship could be used to describe the data. I have revisisted the problem and revived the calculations for 2009 vehicles. I have posted my results on the Magnesium Forum, see link below.
    I would very much like to use your data too, if you can share the Excel file with me. That would give us data for 2005, 2007 and 2009 and would make a good comparison of how fuel economy issue is being addressed on a year-to-year basis.

Leave a Reply