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| Fig. 1: Vehicle city fuel efficiency η (in km/liter) vs. curb mass. As can be seen, there is a strong relationship between weight and efficiency. |
Automakers are under huge pressure to improve fleet fuel efficiency. Consumers, still wary of 2008's record oil prices, are shifting to more fuel efficient vehicles. [1] At the same time, the federal government recently imposed drastically higher efficiency mandates through its Corporate Average Fuel Economy (CAFE) regulations. [2]
In order to reach these new efficiency requirements, some groups claim that car manufacturers will have to cut vehicle weight, which would have a detrimental impact on vehicle safetyat the expense of vehicle safety. [3] The question remains, however, how much of an effect does vehicle weight have on fuel efficiency?
While vehicle weight should largely determine the amount of energy required to accelerate and the energy lost to road friction, there are many sources of energy use and inefficiencies in a car. Air conditioners and radios, for example, all consume energy. According to Gardner and Stern, improving fleet economy from 20 mpg to 30.7 would cut US individual energy usage by 13.5%. [4] The same source claims that frequent tune-ups, proper tire inflation, and using low rolling resistance tires could US per capita energy usage by a total of 6.6%. If these claims are correct, and if tire and engine maintenance and optimization can provide half the energy savings as a 50% increase in total fuel economy, it would imply that vehicle weight is only one of many factors that determines a vehicle's fuel efficiency.
The purpose of this paper is to analyze vehicle data to determine the effect of vehicle curb weight on fuel economy. The analysis looks at available weight and fuel efficiency data for 2009 model year vehicles from Chevrolet, Ford, Chrysler, Toyota, Honda, and Nissan. Unfortunately, reliable sources for these specifications are difficult to come by. Vehicle curb weight is often provided by the manufacturer, who may not be an unbiased source. Fuel efficiency numbers come in the form of EPA estimated miles per gallon (MPG), which some have complained overestimates fuel economy. [5] Furthermore, the data itself is varies depending on the source, with various sources for car specifications having slightly different weight and fuel efficiency numbers.
With the inherent volatility of the data, some may argue that any analyses would be invalid. It seems, however, that any biases and systematic errors in the measurements or in their reporting would effect all car manufacturers equally, allowing general trends to be derived from the data. Ultimately I relied on Kelley Blue Book's online database for curb weight data, and on the epa for fuel efficiency data. [6-7] The resulting analysis shows that, while vehicle weight does play an important role in vehicle economy, vehicle optimizations and hybrid powertrains can substantially improve fuel efficiency for most weight classes.
In determining the effect of vehicle weight on vehicle gas mileage, it is important to first understand the type of relationship that exists between weight and energy consumption.
There are a number of inefficiencies and sources of energy consumption in a vehicle. Two terms, however stand out as being the most significant from a weight perspective: the energy required to accelerate the car, and the energy required to overcome road friction. Since of these factors scale linearly with vehicle weight, it can be said that energy consumption as a whole scales roughly linearly with vehicle weight.
It is important to note that fuel efficiency η is actually a measure of distance per energy. Therefore, fuel efficiency in kilometers per liter (1 MPG = 0.425 kg/l) should scale inversely to vehicle mass. Therefore, for this analysis, weight data and fuel efficiency were related as seen in Eq. (1), where η is EPA estimated fuel economy, M is vehicle curb mass and a, b, and c are fitting parameters.
| η = a M-b + c | (1) |
With (1), it is possible to determine how much of an effect vehicle weight has on fuel efficiency.
From the data, it is clear that vehicle weight has an inverse relationship with fuel economy. As seen in Figs. 1 and 2, fuel economy roughly halves when vehicle weight is increased from 1100 kg to 2700 kg. Therefore, it is safe to say that curb weight has a strong effect on vehicle efficiency. Despite this relationship, a careful analysis of the vehicle data shows that many other factors also substantially impact vehicle fuel efficiency. Specifically, the curve-fits for vehicles' city and highway gas mileage versus weight show a large variation between fuel efficiency in vehicles of roughly the same weight class.
For city mileage, where the root mean sum of residual errors is 0.83, optimizing a 1100 kg car for maximum fuel efficiency has the same effect as reducing vehicle curb weight by 51 kg; optimizing a 2700 kg car for maximum fuel efficiency has the same effect as reducing curb weight by 700 kg. Applying this analysis to highway fuel efficiency provides similar results.
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| Fig. 2: A comparison of highway fuel efficiency η vs. vehicle mass. Once again, there is a strong relationship between weight and fuel efficiency. |
Admittedly, these numbers may are not very impressive for small cars. For large cars, however, merely optimizing a vehicle for fuel efficiency has the same effect as decreasing curb weight by a third. This result shows that vehicle weight is not the most important factor in determining fuel efficiency in large cars.
While simply optimizing a given vehicle can have a substantial effect on fuel efficiency, moving to a hybrid powertrain completely shifts the curb weight to efficiency relationship. For city mileage, at 1350 kg, a hybrid is 90% more efficient than a traditional vehicle, while at 2700 kg, a hybrid is 40% more efficient than a highway vehicle. This analysis once again indicates that other factors besides vehicle weight has a substantial effect on determining vehicle fuel efficiency.
From Figs. 1 and 2 it is clear that vehicle weight has a strong effect on fuel efficiency. A careful analysis of the data, however, reveals that vehicle weight is far from the only factor affecting vehicle fuel efficiency.
For large cars in the 2009 model year, simply optimizing the engine and powertrain for fuel efficiency gives the same efficiency improvements as decreasing vehicle weight by a third. Additionally, in the 2009 model year, switching to a hybrid powertrain makes a vehicle 40-90% more efficient. Therefore, while weight may be an important factor in determining vehicle efficiency, if the efficiency and weight metrics can be trusted, vehicle optimizations can lead to similarly impressive gains in fuel economy.
© Andrew Danowitz. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. S. Carty, "Cars Outsell Trucks in Rough Year," USA Today, 6 Jan 09.
[2] J. Voorhees, "White House Rolls Out Details of Auto Fuel Economy, Emissions Standard," New York Times, 15 Sep 09.
[3] L. Evans, "Does CAFE Kill?" Competitive Enterprise Institute, 17 Jan 02.
[4] Gerald T. Gardner and Paul C. Stern, "The Short List: The Most Effective Actions U.S. Households Can Take to Curb Climate Change," Environnment Magazine, Sep-Oct 2008.
[5] James R. Healey, "Drivers Irked as Mileage Fails to Add Up," USA Today, 18 Aug 04.
[6] Kelley Blue Book.
[7] www.fueleconomy.gov, U.S. Department of Energy.