Fuel Consumption - General Information
The cause of excessive fuel consumption is hard to diagnose and correct because of the potential number of factors
involved. Actual fuel consumption problems can be caused by any of the following factors:
— Engine Factors
— Vehicle Factors and Specifications
— Environmental Factors
— Driver Technique and Operating Practices
— Fuel System Factors
— Low Power/Driveability Problems
Before troubleshooting, it is important to determine the exact complaint and whether the problem is real or
perceived, does not meet drivers expectations. The ‘‘Fuel Consumption - Customer Complaint Form’’ is a valuable
list of questions that can be used to assist the service technician in determining the cause of the problem. The
form must be completed before troubleshooting the complaint. The form can be found at the end of the Trouble-
shooting Symptom section. The following are some of the factors that must be considered when troubleshooting
fuel consumption complaints.
Result of a Low Power/Driveability Problem: An operator will change his driving style to compensate for a low
power/driveability problem. Some things the driver is likely to do are, (a) shift to a higher engine RPM or (b) run
on the droop curve in a lower gear instead of upshifting to drive at part throttle conditions. These changes in driving
style will increase the amount of fuel used.
Driver Technique and Operating Practices: As a general rule, a 1 mph increase in road speed equals a 0.1 mpg
increase in fuel consumption. This means that increasing road speed from 50 to 60 mph will result in a loss of fuel
mileage of 1 mpg.
Environmental and Seasonal Weather Changes: As a general rule, there can be as much asa1to1.5mpg
difference in fuel consumption depending on the season and the weather conditions.
Excessive Idling Time: Idling the engine can use from 0.5 to 1.5 gallons per hour depending on the engine idle
speed.
Truck Route and Terrain: East/west routes experience almost continual crosswinds and headwinds. Less fuel can
be used on north/south routes where parts of the trip are not only warmer, but see less wind resistance.
Vehicle Aerodynamics: The largest single power requirement for a truck is the power needed to overcome air
resistance. As a general rule, each 10 percent reduction in air resistance results in a 5 percent increase in mpg.
Rolling Resistance: Rolling resistance is the second largest consumer of power on a truck. The type of tire and
tread design have a sizeable effect on fuel economy and performance. Changing from a bias ply to a low profile
radial tire can reduce rolling resistance by about 36 percent.
Additional vehicle factors, vehicle specs and axle alignment, can also affect fuel consumption. For additional
information on troubleshooting fuel consumption complaints, see Troubleshooting Excessive Fuel Consumption,
Bulletin No. 3387245.
Troubleshooting Symptoms Charts SectionT-Troubleshooting
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