To assist students in becoming wise energy consumers by understanding how to determine vehicle mile per gallon (MPG); using the MPG data, students should also be able to determine when a vehicle's engine malfunctions.

The student will:
1. Learn to properly calculate fuel-miles-per-gallon (MPG).

2. Learn, through experience, how MPG may vary with driving conditions/habits.

3. Realize that engine malfunctions can be detected through changes in MPG.

Significant increases in fuel economy have taken place in the last decade. See Table 1 for data.


Average Fuel Consumed
per Car
Average Miles Traveled
per Car
Average Miles Traveled
per Gallon of
Fuel Consumed

Gallons Miles Miles
1967 684 9,531 13.9
1968 698 9,627 13.8
1969 718 9,782 13.6
1970 735 10,121 13.6
1971 746 10,121 13.6
1972 755 10,184 13.5
1973 763 9,992 13.1
1974 704 9,448 13.4
1975 712 9,634 13.5
1976 711 9,763 13.7
1977 706 9,839 13.9
1978 715 10,046 14.1
1979 664 9,485 14.3
1980 603 9,135 15.2
1981 579 9,000 15.5
1982 587 9,530 16.3
1983 578 9,650 16.7
1984 553 9,790 17.7
1985 549 9,830 17.9

Clearly, there has been an improvement in the number of miles that a gallon of gasoline can deliver in later model vehicles. A key element in the improvement of fuel economy has been the computer. Since 1981, the majority of vehicles produced have used some form of computer to control part or all of the ignition, the fuel, and the emissions control systems. See illustration A.


Prior to computer control, each system (ignition, fuel, emissions) essentially functioned independently of one another. See illustration B.


If any portion of the non computer controlled system malfunctioned or became out of adjustment, the vehicle would obviously inform the driver by missing, bucking, or delivering less power and performance. With a computer controlled vehicle, minute adjustments are automatically taken care of as the vehicle is driven. Thus as components wear the need for service and maintenance is disguised since the vehicle will not miss, buck, nor deliver significantly reduced power. However, fuel economy will gradually deteriorate to compensate for the wear on various ignition, fuel, and emission system parts. Keeping MPG records can be a valuable asset in determining when a vehicle needs to be serviced. Fleet operators use this method to determine when a vehicle requires servicing. A sample data sheet is displayed in Chart 1.


Date Fuel Purchased Odometer
Mix City, Hwy
or City & Hwy
ex. 8/14 Fill Tank 30,455

ex. 8/17 12.7 30,723









Current Odometer Reading - Previous Odometer Reading = Miles Traveled
Miles Traveled / Fuel Purchased = MPG

To calculate fuel economy, start with a full tank; note the odometer (odometer #1) reading. Drive the vehicle normally and note the use mix (city, highway, or city-highway mixed). When the fuel gauge reaches approximately ¼, refill the tank and note the odometer (odometer #2) reading. Subtract odometer #1 from odometer #2; divide the result by the number of gallons purchased. One significant decimal place is sufficient. Examples: Odometer reads 30,455 when the tank is first filled. The vehicle is driven until the fuel gauge reads ¼ ; the odometer reads 30,723. It took 12.7 gallons to fill the tank again. Subtract the 30,455 from 30,723 and divide the result by the 12.7. [30,723 - 30,455 = 268]

[268 / 12.7 = 21.1 MPG]

The traffic mix -- city and highway -- affects a vehicle's MPG. During idle times (times when the engine is running and the vehicle is not moving) the vehicle delivers 0 MPG. Also, each time the vehicle is accelerated, more power (fuel) is required to bring the vehicle to a given speed. The start and stop driving that characterizes in-city driving is, by its very nature, fuel inefficient. On the highway, speeds are usually steadier resulting in greater fuel economy. The way that the fuel tank is filled can affect fuel economy. Many drivers habitually over fill their tanks, that is they fill them to the very top or "top-it-off". Vehicles built since 1970 have evaporative emissions control systems. Essentially, these systems eliminate fuel-fume emissions as a result of increased ambient temperatures. The design of this system is to vent excessive tank pressures to an activated charcoal canister and store these fumes instead of venting the excess tank pressure and fumes to the atmosphere. These fumes are eventually burned by the engine thus emissions are reduced and fuel is saved. See illustration C.


If the tank is over filled, liquid fuel may be forced into the canister. Repeated over filling can result in permanent damage to the canister. As a result, the vehicle's fuel system may need to compensate for the nonfunctioning charcoal canister. To properly fill the tank, add fuel until the automatic shutoff stops the flow. A slower than full flow rate will prevent premature shutoff. Do not continue to fill beyond two shutoffs in close succession.

1. Use your own car or the family car to perform the MPG calculation.

2. Use the work sheet to collect data.

3. Fill tank and note the odometer reading.

4. Drive the vehicle as it is normally driven. Note general driving mix during tank use; drive the vehicle normally until there is approximately 1/4 fuel left in the vehicle's tank.

5. Refill; note odometer reading.

6. Repeat 3 thru 5 three times. Subtract the first odometer reading from the third fillup odometer reading. Add the fuel used for all three tanks. Divide the total mileage driven by the total fuel used for the three tanks. This is BASE AVERAGE (assuming a consistent city/highway mix).

7. Note any specific differences when the mix was either city or highway dominant.

8. Keep records or monitor MPG at least once per month. If there are any significant (.5 MPG) change for three consecutive tankfuls, determine the cause for that decrease.

1. Explain why you think city and highway MPG's differ.

2. Compare the MPG you received in city and on highway or city/highway combined with the MPG of three classmates. Try to determine the reasons for the variances such as engine size, options (air conditioning, power steering, etc.), vehicle size, speeds driven, driving habits (idle time in the fast food or bank teller lines, hard/easy acceleration and braking habits).

3. (Optional) Write a short paragraph on what was learned about MPG.

Place a "T" before the statements that are true and an "F" before the statements that are false. After each false statement, explain why it is false.

_______1. The average MPG has steadily increased since 1967.

_______2. Computer controlled engines are able to compensate for the minor wear of engine parts such as spark plugs, distributor caps and rotors, or the deterioration of air filters and PCV valves.

_______3. Keeping a record of the fuel consumed and the MPG delivered can be of assistance in determining engine malfunctions.

_______4. To properly calculate MPG, divide the gallons consumed by the miles driven.

_______5. To get an accurate MPG check, fill the tank to the top of the filler neck on each fuel fill up.

_______6. Fuel fumes that are stored by the evaporative emissions canister are reintroduced into the engine and burned.

_______7. The traffic mix--highway, city, or combination highway and city--can affect a vehicle's MPG.

Teacher's Notes
This is a simple but important exercise. Students must know how to accurately calculate fuel MPG before attempting the other vehicle fuel economy guides. It is important that a three tank average be used as the base for fuel MPG. The three tank method will correct any variances in filling the tank. Also, the students need to experience the difference between city, highway, and mixed city/highway driving. In the opening discussion, students should come to realize that rapid starts and stops, extended idle times, and erratic driving speeds (the constant acceleration and deceleration) are the major factors in city fuel eaters.

Answers to Information Check Questions:
1. False. From 1967 until 1973 the average vehicle MPG declined; not until 1977 did the average vehicle MPG equal to the 1967 MPG.

4. False. To calculate MPG, divide the miles driven by the gallons used.

5. False. Do not fill the tank to the top of the filler neck; this can cause the evaporative emissions canister to fail.

Recommended Reading:
Shell Oil Company. How to Get More Miles Per Gallon. The Better Mileage Book. 1991.

DriveWise*. Keep Track of Your Savings. Arizona Energy Office of the Department of Commerce. n.d.

Comments or questions to: TechAsmt@LA.GOV

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