Understanding Fuel Economy Ratings

Recently, I had a long-time customer come in with her new car, complaining about her gas mileage. It was a 2000 model year car with about 9000 miles on it. She said that the window sticker said the gas mileage was 27 miles per gallon in the city and 34 on the highway. She said that the best she ever got with it was 19 mpg. She wanted to know if there was something wrong with her car and whether she should bring it back to the dealer to have it looked at.

Like many other people, she misunderstood what those numbers actually mean. It's not only important to have information; it's just as important to understand what that information means. In this case, the numbers are not going to tell you what your gas mileage will be; it is just an indication of the relative fuel economy of the vehicle.

Okay, so what do those numbers on the window sticker mean and how do they arrive at them?

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 comparison of current vehicle fuel economy to the EPA fuel economy estimates is to let you compare apples to apples.

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 ensure that the results can be compared with confidence.

The EPA GAS MILEAGE GUIDE also points out that the 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 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).

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.