Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa

The Influence of Rural Road Quality on Vehicle Fuel Consumption

Jerry Fruin and Dan Halbach

University of Minnesota,
Department of Applied Economics,
231 Classroom Office Building,
1994 Buford Avenue,
St. Paul, Minnesota 55108.

The purpose of this research is to determine the difference in fuel consumption of a vehicle due to road surface type and quality. Rural road sections in three Minnesota counties (Anoka, Dakota, and Chisago) were studied. The road surfaces included new bituminous, gravel surfaces of varying condition and aggregate composition, and one earth surface. The use of an in-vehicle inertial dynamometer to measure "work" was demonstrated. Key words: surface, energy, gravel, dynamometer, road.

Accurate knowledge of increased cost due to the quality of road surface is very important if we are to determine the most cost-effective methods and procedures for deciding when, where, and to what extent rural roads should be maintained or upgraded. The existing information often used by various studies including Baumel (1) is based on data from Robley Winfrey's (2) work at the Iowa Engineering Experiment Station in the 1920s and 1930s (3). Current vehicles (and rural roads) are not comparable to those that existed when the Winfrey studies were done. The authors' previous work (4) on a computer simulation of a 1,500-square-kilometer area with three categories of traffic (agricultural marketing, personal travel, overhead) made clear that surface type and the associated variable cost assumptions are important. A project was proposed to the Minnesota Local Road Research Board (made up of county engineers) to measure fuel consumption using replicated field tests over measured courses. While attempting to design such a study, we discovered the existence of an onboard inertial dynamometer. While this technology was developed and used for racing applications, we evaluated and used this technology to determine actual road surface energy relationships. The resulting cost estimates and methodology using these new technologies should be useful to researchers and officials evaluating alternative investment, disinvestment, and maintenance strategies for funding roads.

A DYANAFACT onboard dynamometer and computer developed by Banks Technologies was installed in a 1994, H ton, 4 x 4 extended cab pickup. Baseline chassis dynamometer laboratory runs were done to determine rear wheel horsepower to check and calibrate the on board dynamometer. The DYANAFACT system computes horsepower based on a vehicle's velocity (kph from a calibrated speed sensor), acceleration (g forces monitored by means of an accelerometer), mass (vehicle weight), and vehicle drag characteristics. Given these parameters, a microprocessor can mathematically calculate horsepower. Data logging 20 times per second records time stamp, horsepower, g force, mph, and distance traveled. An analysis of horsepower requirements is presented by vehicle speed and tire pressure for different surface types.

The road surfaces included new bituminous, gravel surfaces of varying condition and aggregate composition, and one earth surface. All sites were at least three kilometers in length and in general very flat. The paved site was in Anoka County at 149th Avenue near the city of Ham Lake. The three gravel sites were in Chisago County. The highest quality segment was on Chisago Co. 70. This is a wide, well maintained surface. Chisago Co. 69 site has less gravel and is found in an area with peat soils and sod farms. The last site was hard packed and had very little gravel on the surface on 310th Street near Taylors Falls. The earth surface site located at 190th Avenue in Dakota County is a posted as a minimum maintenance road signed with "travel at your own risk."

A number of parameter calibrations were carried out before each set of runs on a daily basis. Vehicle weight was checked with individual scales at each wheel to check distribution and adjust ballast to maintain consistent weight. Tire pressure was set 50 psi cold. Speed was held constant with the vehicle electronic speed control. Data was logged in both directions at various speed over a distance of 1.6 kilometers. Figure 1 shows how horsepower requirements changed with speeds for three surfaces. It is interesting to note the earth road required less horsepower than gravel. A tire pressure change from 50 psi to 35 psi on the Chisago Co. 69 segment increased horsepower from 6.4% to 10.9%. See Table 1. Table 2 shows power requirements when pulling a two-axle trailer at two weight levels. Horsepower-hour can be converted to BTUs by multiplying by 2544.4336 (5). Gasoline has a BTU content of about 125,000 so with engine efficiency factors, fuel consumption can be estimated.

TABLE 1 Change in Required Horsepower from Tire Pressure Change on a Gravel Surface

Speed in Kilometers per Hour (MPH)
Tire pressure 48.3 (30) 64.4 (40) 80.5 (50)
50 psi 10.9 19.4 31.6
35 psi 11.6 20.9 34.5
% increase 6.4 7.7 10.9
TABLE 2 Horsepower Requirements of Various Weight on a Gravel Surface

Speed Kilometers per Hour (MPH)
weight 48.3 (30) 64.4 (40) 80.5 (50) (60)
2449 kg 10.9 19.4 31.6 47.9
3900 kg 24.3 41.7 64.9 92.5
5262 kg 31.7 59.2 82.8 NA


This research is supported by funding from the Minnesota Local Road Research Board, Minnesota Department of Transportation, Center for Transportation Studies at the University of Minnesota, Minnesota Agricultural Experiment Station. Minnesota Agricultural Experiment Station No. 22,304. Special thanks to the county engineers who assisted in the project.


  1. C.P. Baumel, S. Hanson, C. Hamlett, and G. Pautsch. Vehicle Travel Cost on Paved, Granular and Earth Surfaced County Roads. Transportation Research Forum, Proceedings Twenty-sixth Annual Meeting, Washington, D.C., 1985, pp. 401–406.
  2. R. Winfrey. Economic Analysis for Highways. International Textbook Company, Scranton, Pennsylvania, 1969.
  3. R. Winfrey. Automobile Operating Cost and Mileage Studies. Iowa State College of Agriculture and Mechanic Arts, Official Publication, Vol. 30, No. 8, 1931.
  4. J. Fruin and D. Halbach. Rural Roads, Investment and Disinvestment in a Minnesota County. University of Minnesota, Department of Agricultural and Applied Economics Staff Paper P92-25, St. Paul, Minnesota.
  5. J. Lawlor. Auto Math Handbook: Mathematical Calculations, Theory, and Formulas for Automotive Enthusiast. HPBooks, Los Angeles, California, 1992.

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