Wednesday, September 17, 2008

What is the average vehicle mass in grams?

Today we will discuss mass. Grams are a metric unit of measure of mass. The US EPA reported that the fuel efficiency of the average vehicle sold in 2007 was the same as the average of a vehicle sold in 2006 at 20.2 miles per gallon almost ten percent lower than the average for the year 1987. This means we have been going backwards since 1987 in our fleet efficiency thanks to the size and mass of the average vehicle sold in 2007. Therefore we can thank grams for our current energy problems. The interesting data on fleet fuel efficiency can be found at

It is interesting that the average mass of vehicles sold in the USA has increased significantly since 1987 from 3,221 pounds to 4,144 pounds or 1,873,088 grams. The average horsepower of a vehicle has almost doubled since 1987 from 118 to 223 horsepower. The heaviest horse in history had a mass of 3,360 pounds or about the mass of the average vehicle sold in the USA in 1987. This horse named Sampson lived in England in 1846, and had he not been gelded he would have weighed even more. The acceleration of the average vehicle measured in the time taken to get from zero to sixty MPH has decreased from 13.1 seconds to 9.6 seconds in the past twenty years. The fuel economy of the average vehicle has decreased from 22.0 mpg to 20.2 mpg because of the change in mass, horsepower and the proliferation of trucks (SUVs included) from only 28% of vehicles sold in 1987 to 49% of the vehicles to be sold in 2007.

If we all remember a previous Green Machine episode explaining the Newton’s laws of motion we all know that the force that is needed to be applied to an object to yield a set acceleration is directly proportional to the mass ( F=MA ). In accelerating cars from zero to sixty there are some other forces (resistance) to be overcome such as wind resistance and rolling resistance but for the moment will simply deal with old Isaac’s second law. The mass of the vehicles has increased in the past twenty years by 29%. Acceleration increased by 36% so accounting for increased mass and increased acceleration one would have expected the required force from the engine to have increased by 75% which is in line with the added horsepower in the 2007 models. With all this increased mass, faster acceleration and more powerful engines how did we still achieve only an 8% reduction in fuel efficiency? This can be answered by the improved efficiency of the later model engines. Primarily as a result of fuel injection, multi valve engines, and variable valve timing that were not available back in the old days. These improvements all relate to how the fuel is burned in the engineIn the future we will have higher compression engines, direct fuel injection, and of course smaller and lighter vehicles. We can thank the Governator for the proliferation of SUVs as his movie Kindergarten Cop started the Hummer rage. My prediction is that within a decade the average mass of a vehicle will drop to value we had in 1987 and the average fuel efficiency will be close to 30 mpg. Also within a decade we will have long forgotten Arnie.


  1. Lindsay,
    That was a very interesting report you found. It's also very sad that 2007 was the first time in 20 years that the average fuel economy increased for two consecutive years. But, who's to blame? Car companies for making heavy SUVs or the public for buying them?
    As gas prices go up, it seems that SUV and truck prices go down. Should car companies make lighter SUVs with better efficiency or should Americans buy cars like the Prius?


  2. Hi Ryan

    The Hummer was a Bummer and it will go the way of the Rambler. The US auto companies will market small but fast cars and we will all think this is great. We will also have station wagons for taking the kids to soccer or maybe we will all just walk a little more. Enjoy Italia