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Anyone driving his/her vehicle on a regular basis is concerned about its fuel economy While many older vehicles were not very fuel efficient, it is in the interest of both the owner and the environment to ensure that the vehicle consume as little fuel as possible It is useful to keep a fuel consumption records to have a baseline for the vehicle's expected fuel economy and to have some indication of developing problems.

Often drivers recall their fuel economy from memory but, without circumstances and data, this can be misleading Some factors that could make today's fuel economy different than historical reality are:

  • Recollection of only one particular trip
  • Incorrect fuel economy calculations
  • Calibration of odometer (tires/axle gear/odometer gear)
  • Deterioration in mechanical condition
  • Variation in gas tank fill amount level
  • Variation in fuel's energy content [eg, 100% gasoline vs E10 (gasoline with 10% ethanol)]
  • Variation in average vehicle speed

While it is useful to have a historical data as a baseline, accurate data is often difficult to obtain Without a record of the vehicle's fuel consumption history, it is difficult to have a good idea what its typical fuel consumption should be. US EPA fuel mileage ratings started in 1972 and were based on FTP-72 (FTP - Federal Test Procedure) FTP-72 was originally modeled on rush-hour driving in Los Angeles and did not often correspond well with real-world fuel economy.

Keeping modern fuel consumption records is a good way to see trends in your vehicle's fuel economy. Spreadsheets are readily available and are an excellent means of tracking fuel economy, especially when their built-in charting capabilities clearly show trends. Use a GPS with a built-in trip odometer to calibrate the vehicle's odometer because modern metric tires may not have the same dynamic (rolling) diameter as OEM tires.

Many people often recall that big cars with V8 engines would often have better fuel economy than compact cars with inline 6-cylinder engines It is difficult for larger cars to have better fuel economy for 2 reasons: frontal area (physical size) and drag coefficient (how streamlined). For the same vehicle and transmission (eg, automatic vs manual), a smaller engine should theoretically have better fuel efficiency because of reduced internal friction (mechanical - piston rings & bearings, pumping loss).

Pumping loss is the work the engine must do to draw a fuel mixture across a nearly closed throttle valve and is reflected by high manifold vacuum Reducing manifold vacuum (ie, having it closer to atmospheric pressure) is commonly done by using highway gears (lower numerically) in the axle and overdrive transmissions Newer, emission-controlled vehicles also use EGR (Exhaust Gas Recirculation) to reduce pumping loss in addition to NOx control.

Some basic things to check to ensure optimum fuel economy:

  • Tune-up items (initial timing, dwell, idle mixture & speed, tappet clearance, etc)
  • Condition of air & fuel filters
  • Condition of choke system
  • Condition of ignition system (points & condenser, cap, rotor, wires, spark plugs)
  • Condition of centrifugal & vacuum advance mechanisms
  • Condition of emission control systems (as equipped):
    • Manifold Heat Control System
    • PCV (Positive Crankcase Ventilation) System
    • EGR (Exhaust Gas Recirculation) System
    • Heated Air System
    • Evaporation Control System
  • Tire pressure
  • Engine operating temperature
  • Eliminating unnecessary weight (ie, junk in the trunk)

If the basic items are within specifications and working properly, some more advanced things to check are:

  • Engine compression & variation between cylinders
  • Wheel alignment
  • Dragging brakes
  • Timing mark verification (harmonic balancer slippage)

If everything is working properly, the following are some things that could further improve your fuel economy:

  • Higher compression (new pistons/rings, modified cylinder head)
  • Carburetor rejetting (check for optimum fuel mixture with a wide-band O2 sensor)
  • Timing advance optimization (requires dynamometer work)
  • Engine speed reduction (highway gears in axle, overdrive transmission)
  • Upgrade to high energy electronic ignition