Fuel - How Fuel Economy Is Determined
Group Ref.: Engine Fuel & EmissionBulletin No.: 306502
Date: October, 1993
SUBJECT:
INFORMATION ON FACTORS THAT AFFECT FUEL ECONOMY
MODELS:
1993 AND PRIOR YEARS/ALL MODELS
This bulletin is being revised to add Cadillac information at the end of the bulletin. Previous divisional publication numbers were:
GM POWERTRAIN RELEASE NAME:
LARRY THATCHER, NO 8-446-4825
BACKGROUND INFORMATION:
EPA fuel economy estimates are posted on the fuel economy label of all new vehicles. The only intended use of these values is for comparison among the different vehicles. Fuel economy estimates are generated from data taken during a laboratory test using pre-production prototype vehicles under extremely controlled conditions using a professional driver, with the vehicle operating on an instrument similar to a treadmill. The comparisons of current vehicle fuel economy to the EPA fuel economy estimates is a misuse of the information and should be discouraged.
The EPA GAS MILEAGE GUIDE, available at each dealership, points out that the actual mileage when driving a vehicle may differ considerably from the estimated mileage. The guide also describes how vehicles are tested under identical conditions to insure the results can be compared with confidence.
The EPA GAS MILEAGE GUIDE also points out that city fuel economy estimate simulates a 7.5 mile, stop-and-go trip with an average speed of 20 mph. The trip takes 23 minutes and has 18 stops. About 18 percent of the time is spent idling, as in waiting at traffic lights or in rush hour traffic. Two kinds of engine starts are used - the cold start, which is similar to starting a car in the morning after it has been parked all night - and the hot start, similar to restarting a vehicle after it has been warmed up, driven and stopped for a short time.
The test 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 trip and averages 48 mph. The test is run from a hot start and has little idling time and no stops.
The EPA GAS MILEAGE GUIDE explains that the actual test results are adjusted downward to arrive at the estimates used in the booklet and on the labels. City estimates are lowered by 10 percent and the highway estimate by 22 percent from the laboratory test results. The guide also points out that traveling at higher speeds lowers fuel economy and traveling at 65 mph instead of 55 mph lowers fuel economy over 15 percent.
FACTORS THAT AFFECT FUEL ECONOMY.
Axle Ratio
Numerically lower 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 5 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 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 ambients 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 which gets 25 to 30 MPG on 100 percent gasoline.
Using fuels of a lower octane than the vehicle was calibrated to will cause increased "KS" Knock Sensor 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 fillups increase.
Green Engine
New vehicles have not yet had an opportunity for the engine to break in, (rings to seat ... ). A typical engine will take 3 to 5 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 backglass ... ) 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 unperceptible 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 ride.
Transmission
On 4-Speed automatics, it is possible to drive the vehicle in 3rd 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 linkage 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 3rd gear rather than overdrive at highway speeds typically results in a 3 to 5 MPG penalty.
[1][2]Torque Converter Clutch operation is essential for good fuel economy. A non-locking torque converter typically results in a 1 to 2 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, luggage will decrease fuel economy.
Vehicle Wind Resistance
More wind "DRAG" means less fuel economy. Thus, hang-on luggage carriers, cat toppers, open windows and/or open trunk mean less fuel economy. (See "Driving Habits").
CADILLAC INFORMATION BEGINS HERE:
The following information is included in all 1994 Cadillac Owner's Manuals and applies to all years and models of Cadillacs.
Your fuel economy (miles per gallon or liters per 100 kilometers) can vary depending on how your Cadillac 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 which was attached to your new Cadillac when it was delivered and in the Gas Mileage Guide which 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. Cadillac tests prototypes of new vehicles and submits the results to the EPA. The EPA then confirms the accuracy of the figures provided by Cadillac. The vehicles are driven by a professional driver under controlled laboratory conditions on an instrument similar to a treadmill.
NOTE:
All of these tests are done with the A/C 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.
Even though these figures are adjusted, they still represent what the average driver will get. Your fuel economy may be significantly higher or lower, depending on how, when, and where your vehicle is driven. Here are some things you can do to increase fuel economy:
^ Combine errands into one trip.
^ Turn the engine off rather than letting it idle for more than a minute.
^ Have your Cadillac serviced as described in the maintenance booklet.
^ Keep tires inflated to recommended pressures.
^ Anticipate traffic stops.
Here are some things that will lower fuel economy:
^ Quick acceleration.
^ Traveling at higher speeds. Traveling at 65 mph instead of 55 mph lowers fuel economy by 15 percent.
^ Carrying unnecessary weight in the vehicle.
^ Revving the engine. This is not necessary for your vehicle.
^ Operating your vehicle with the suspension out of alignment or with the wheels and tires out of balance.
^ Use of electrical accessories which require high amperage when they are not needed.
Even things beyond your control, such as weather conditions, affect your fuel economy. Driving up steep hills, in rain or snow, and into a strong wind will lower fuel economy. In the lower left corner of the fuel economy window sticker for your vehicle is the range of city and highway fuel economy you can expect to get from your Cadillac. If you are doing everything to raise your fuel economy but are still not within this range, your vehicle may need service. Collect as much information about your fuel economy as you can find (miles driven, gallons of fuel used, etc.) and provide it to your dealership for their review and assistance.