Fuel economy for vehicles (city, highway and combined) is measured under controlled conditions in a laboratory using a series of tests specified by federal law.The results are displayed for the consumer on the "new car window sticker."
Estimating MPG with Laboratory Tests
In the laboratory, the vehicle's drive wheels are placed on a machine called a dynamometer. The "dyno" simulates the driving environment much like an exercise bike simulates cycling.
Engineers adjust the amount of energy required to move the rollers to account for wind resistance and the vehicle's weight.
On the dyno, a driver runs the vehicle through standardized driving routines called cycles or schedules. These cycles simulate "typical" trips in the city or on the highway.
Each cycle specifies the speed the vehicle must travel during each second in the test.
Measuring Fuel Use
For vehicles using carbon-based fuels (e.g., gasoline, diesel, natural gas, etc.), a hose is connected to the tailpipe to collect the engine exhaust during the tests.
The carbon in the exhaust is measured to calculate the amount of fuel burned during the test. This is more accurate than using a fuel gauge.
A different method is used for vehicles that run on non-carbon fuels, such as fuel cell vehicles and electric vehicles.
The Inside Story
Although the EPA establishes the tests that yield the fuel economy figures, it doesn't conduct the tests itself. This is because it lacks the budget, equipment and manpower to test the hundreds of individual models with unique engine and transmission combinations that automakers produce each year.
Instead, the agency gives its test protocols to the auto companies and lets each test its own vehicles- usually pre-production proto-types. In the past, this sometimes created inflated mileage estimates since it accepts, as true, the "EPA estimated" fuel-efficiency numbers each car company submits. To keep the industry honest, the agency confirms about 15%–20% of them through their own tests at the National Vehicles and Fuel Emissions Laboratory.
A key element in assessing the EPA rating for a vehicle's average fuel economy (EPA combined) is the split between highway and city driving. Almost all cars and trucks deliver better fuel economy while cruising at 55 mph on the open highway than they do while stopping and starting at low speed on city streets.
What may be surprising to many is the fact that ever since the EPA modified its testing process and applied those changes to the 2008 model year, its ratings aren't really all that far off the real-world mpg that consumers get. While there are lots of people who cannot under any circumstances get their vehicles to come close to the official ratings, there are also lots who regularly meet or exceed them.
Dan Edmunds, Edmunds.com's director of vehicle testing, says that the cars and trucks that are most likely to significantly fall short of their EPA combined average ratings are those that are under-powered- (e.g., a big SUV with an optional, downsized four-cylinder engine instead of a standard V6).
Fuel-efficiency researchers David Greene and Zhenhong Lin at Oak Ridge National Laboratory, in Tennessee, recently compared EPA estimates and "Your MPG" reports and found no evidence of over-performance bias in the government's numbers. But researchers did identify the following six factors that could cause a vehicle's real-world fuel economy to vary significantly from its EPA rating:
1. The amount of city, stop-and-go driving. An excessive amount could reduce mileage as much as 27%.
2. Individual driving style. "Calm" drivers, those motorists who don't accelerate rapidly and who avoid unnecessary lane changes, get 35% better fuel economy than other drivers.
3. Use of the air conditioner. Can reduce mileage up to 14%.
4. Vehicle size and weight. Can reduce mileage up to 15%.
5. Fuel type. Most gasoline in the U.S. today is 8-10% ethanol, but the EPA does its tests with 100% gasoline in the tank. The use of ethanol to increase the amount of oxygen in gasoline for better combustion can reduce fuel efficiency by around 2% all by itself.
6. The region where the vehicle is driven. Hot weather and mountainous conditions can reduce mileage up to 12%.
Challenger Fuel Estimates
Here are the EPA’s gas mileage estimates for the Challenger from 2008 to 2018 (all models).