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