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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...

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.

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.

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.

On four-speed automatics, it is possible to drive the vehicle in third gear rather than "overdrive" and not perceive it. Typically, this condition occurs when the shift indicator or the shift linkage/detent is misadjusted. Misadjusted shift linkages can also result in improper signals to the ECM, which can result in less spark advance, and results in a drop in fuel economy.

Driving a vehicle in third gear rather than overdrive at highway speeds typically results in a three to five mpg penalty.

Torque Converter Clutch operation is essential for good fuel economy. A non-locking torque converter typically results in a one to two mpg penalty at highway speeds.

Vehicle Weight:
Each 125 lbs. of additional weight results in a .3-MPG loss of fuel economy. Thus, additional passengers and luggage will decrease fuel economy.

Vehicle Wind Resistance:
More wind "drag" means less fuel economy. Thus, hang-on luggage carriers, car toppers, open windows and/or open trunk mean less fuel economy.

» Part 1       » Part 2       » Part 3       » Part 4
Additional Information provided courtesy of and Warranty Direct
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