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Understanding Fuel Economy Ratings

    What do those numbers on the window sticker mean and how do they arrive at them?

» Understanding Fuel Economy Ratings
» Factors That Affect Fuel Economy
» More Factors That Affect Fuel Economy
» How The Numbers Are Determined

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.

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."

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).

» Part 1       » Part 2       » Part 3       » Part 4
Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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