Understanding Fuel Economy Ratings
Transmission:
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 an open trunk mean less fuel economy.
How The Numbers Are Determined...
Your fuel economy (miles per gallon or liters per 100 kilometers) can vary depending on how your car is driven. Several vehicles like yours have been driven through a standard test, and their actual fuel economy was recorded. These readings were adjusted and printed on the fuel economy window sticker that was attached to your new car when it was delivered and in the Gas Mileage Guide that is available from your dealership.
The fuel economy estimates are based on results of tests required by the U.S. Environmental Protection Agency (EPA). These tests are used to certify that vehicles meet the federal emissions and fuel economy standards. Manufacturers tests prototypes of new vehicles and submit the results to the EPA. The EPA then confirms the accuracy of the figures provided by the manufacturer. A professional driver under controlled laboratory conditions drives the vehicles on an instrument similar to a treadmill.
NOTE:
All of these tests are done with the air conditioning off and all other driver-controlled electrical loads off. These procedures ensure that each vehicle is tested under identical conditions.
There are two different fuel economy estimates for each vehicle: one for city driving and one for highway driving. To develop these two estimates, separate tests are used to represent typical everyday city and rural driving.
The test used to determine the city fuel economy estimate simulates a 7.5-mile (12 km) stop-and-go trip with an average speed of 20 mph (32 km/h). The trip takes 23 minutes and has 18 stops. About one-fifth of the time is spent idling as in waiting at traffic lights or in rush-hour traffic. Two kinds of engine starts are used: a cold start, which is similar to starting a car in the morning after it has been parked all night, and a hot start, which is similar to restarting a vehicle after it has been warmed up, driven, and stopped for a short time.
The test used to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile (17 km) trip with an average speed of 48 mph (77 km/h). The test is run from a hot start and has little idling time and no stops.
To assure that the fuel economy numbers are most useful for consumers, the EPA adjusts these laboratory test results to account for the difference between controlled laboratory conditions and actual driving on the road. The laboratory fuel economy results are adjusted downward to arrive at the estimates on the fuel economy window sticker and the Gas Mileage Guide. The city estimate is lowered by 10 percent and the highway estimate is lowered by 22 percent from the laboratory test results. Experience has proven that these adjustments make the mileage estimates correspond more closely to the actual fuel economy realized by the average driver.
Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved