Understanding Fuel Economy Ratings

   

Factors that affect fuel economy...

   Axle Ratio:
Numerically ower axle ratios generally produce better highway fuel economy. The exception to this is if the engine is "working" exceptionally hard (heavy vehicle loads pulling a trailer, small engine in a large vehicle, etc.). In these cases, a numerically higher axle may provide better fuel economy. Numerically higher axle ratios will also tend to provide more fuel economy in congested city traffic and stop-and-go conditions.

   Brakes:
Brake drag (even a minimal amount undetectable by coasting) can have a significant negative impact on fuel economy. Pull upward on the brake pedal to assure that the stoplight switch and cruise switch at the brake pedal are full and properly adjusted. A "click" sound when the pedal is pulled upward indicates that the switch was improperly adjusted. This causes the front brake pads to lightly rub the rotors, causing a fuel economy loss, without generating excessive heat or brake pad wear.

   Driving Habits:
Frequent short trips (less than five miles), especially in cooler ambient temperatures (less than 65 degrees), will necessitate fuel enrichment on start-ups, especially after "soaks" with the engine off for approximately a half an hour or more.

   Frequent accelerator pedal movement while driving will reduce fuel economy because of fuel enrichment during the periods of acceleration. Under such driving conditions, the torque converter clutch (TCC) also disengages, contributing to fuel economy losses. Prolonged idle periods reduce fuel economy, especially in cold ambient temperatures when vehicle is allowed to "warm up."

   Fuels:
Oxygenated fuels, with methanol and/or ethanol blended into the gasoline, have lower energy and thus reduce fuel economy. Typically, there is about a 1-mpg penalty for a vehicle that gets 25 to 30 MPG on 100 percent gasoline.

   Using fuels of a lower octane than the vehicle was calibrated to will cause increased "Knock Sensor (KS)" system activity. This will result in a net decrease in spark advance and thus poorer fuel economy. Using fuel of a higher octane than the vehicle was calibrated for WILL NOT increase fuel economy.

   Variations in how much fuel is added to the fuel tank during re-fueling can greatly affect calculated fuel economy. These effects decrease as the distance traveled and the number of tank fill ups increase.

   New Engines:
New vehicles have not yet had an opportunity for the engine to break in (rings to seat, etc.). A typical engine will take three to five thousand miles to break in, and during this time period a gradual increase in fuel economy can be expected.

   Parasitic Loads:
Air conditioning and/or electrical loads (headlights, heated back glass, etc.) also result in lower fuel economy (typically less than 1 mpg difference, each 10 Amps takes approximately .4 mpg).

   Road Conditions:
Road surface condition impacts fuel economy. Gravel and/or pot holed roads decrease fuel economy. Hills (vs. level terrain) also negatively impact fuel economy. Even gradual imperceptible increases in elevation result in real measurable decreases in fuel economy. Similarly, driving in the rain or snow decreases fuel economy.

   Suspension:
Vehicle suspension misalignment can cause poor fuel economy. Check all four tires for abnormal and/or premature tire wear. New tires, tire rotation, and/or front-end alignment may be required to correct fuel economy.

   Tires:
Performance tires and/or tires with larger "contact areas" (like 60 series aspect ratio), can cause as much as 3-mpg lower fuel economy when compared to hard "thin" tires. Find out if the tire size currently on the car is the same as original equipment. Replacement tires taller than original equipment tires cause the odometer to read LESS THAN actual distance traveled. This will result in lower calculated fuel economy than actual fuel economy.

   Tire Pressure:
Harder tires (more air pressure, or different tire compositions) result in better fuel economy. Do not exceed maximum pressure as labeled on the tire, typically 30-35 psi. The disadvantage of this is that the greater the tire pressure, the harsher the vehicle rides.

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