How the EPA Measures Fuel Economy

01 Introduction

The miles per gallon (MPG) number on a new vehicle's window sticker comes from a specific, regulated testing process managed by the U.S. Environmental Protection Agency. Understanding how this process works helps explain why your real world fuel economy may differ from the sticker, and why the EPA rating remains the best standardized comparison across vehicles.

This article explains the EPA's testing methodology entirely from official government documentation. For the broader view of what driving costs in America, see our overview. It covers the five test cycles, the 2008 methodology update, the combined rating calculation, the MPGe metric for electric vehicles, and the documented factors that cause real world variation.

02 The Five EPA Test Cycles

EPA fuel economy testing uses five standardized driving patterns, each run on a chassis dynamometer (essentially a treadmill for cars) in a laboratory. The vehicle never drives on actual roads. Instead, a trained driver follows a precise speed and acceleration trace on a monitor while the dynamometer simulates road resistance and vehicle weight.

The original two cycles, the FTP (city) and HWFET (highway), were developed in the 1970s. Three additional cycles were added in the 2008 methodology update to account for high speed driving (US06), air conditioning use (SC03), and cold weather operation (Cold FTP). These three cycles penalize fuel economy, making the label values more realistic.

03 From Lab to Label

Raw dynamometer test results are not placed directly on the window sticker. The EPA applies adjustment factors that reduce the lab numbers to better match real world driving. Before 2008, a simple percentage reduction was applied (10% for city, 22% for highway). The 2008 update replaced this with a more sophisticated five cycle calculation.

Under the current (post 2008) system, the five cycle results are combined using weighted formulas. Manufacturers can choose a simplified calculation (using only FTP and HWFET with larger adjustment factors) or the full five cycle method. Most manufacturers use the five cycle method because it tends to produce slightly higher label values for their specific vehicles.

04 City vs Highway Ratings

The EPA provides three MPG numbers on every new vehicle label: city, highway, and combined. The combined rating uses a weighted average of 55% city and 45% highway, reflecting the EPA's estimate of typical American driving patterns.

The combined rating uses a harmonic mean, not a simple average. This matters because MPG is a ratio. A vehicle rated 25 city / 35 highway does not get 30 combined. Using the harmonic mean formula:

The combined value (28.7 MPG) is lower than the simple average (30.0 MPG) because fuel consumption is linear, not fuel economy. A vehicle that gets half the MPG uses twice the fuel, so the less efficient city driving has a disproportionate impact on the overall average.

05 MPGe for Electric Vehicles

Electric vehicles do not burn fuel, so the EPA created the MPGe (miles per gallon equivalent) metric to allow direct energy efficiency comparisons between EVs and gasoline vehicles. The conversion factor is based on the energy content of gasoline:

A typical efficient midsize EV sedan, for example, consumes about 25 kWh per 100 miles. Converting: 33.7 / 0.25 = 135 MPGe. This means the vehicle uses the energy equivalent of about 0.74 gallons of gasoline per 100 miles, far less than even the most efficient gas car.

However, MPGe does not directly translate to fuel cost. The cost to "fuel" an EV depends on local electricity rates, which vary from about 10 to 30 cents per kWh across the U.S. A high MPGe vehicle is energy efficient, but whether it is cheaper to operate depends on the ratio of electricity prices to gasoline prices in your area.

06 Why Your Mileage Varies

The EPA explicitly states that individual fuel economy will vary. Their testing documentation identifies several factors that cause real world MPG to differ from the label, along with quantified impact ranges:

• Aggressive driving (rapid acceleration, hard braking): reduces MPG by 15% to 30% on the highway and 10% to 40% in the city.
• Driving speed: fuel economy drops rapidly above 50 mph. Each 5 mph above 50 adds roughly $0.24 per gallon to fuel cost (at 2025 prices).
• Cold weather: conventional vehicles lose 15% to 24% of their fuel economy in 20°F weather. EVs can lose 25% to 50% of their range.
• Air conditioning: reduces fuel economy by up to 25% in stop and go driving.
• Roof racks and cargo: a loaded roof rack reduces fuel economy by 2% to 8% in city driving and 6% to 17% on the highway.
• Tire pressure: for every 1 PSI drop in all four tires, fuel economy decreases by about 0.2%.
• Engine oil grade: using the manufacturer recommended grade improves fuel economy by 1% to 2% compared to heavier oil.

07 Data Sources

Every data point in this article comes from a U.S. government source. No proprietary, estimated, or third party data was used.

1. EPA Fuel Economy Testing and Labeling. 40 CFR Parts 86 and 600. Federal Test Procedure (FTP-75), Highway Fuel Economy Test (HWFET), Supplemental Federal Test Procedure (US06, SC03, Cold FTP).
2. EPA Final Rule: Fuel Economy Labeling of Motor Vehicles. Federal Register Vol. 71, No. 248, December 2006. Effective model year 2008. Established the five cycle methodology.
3. EPA FuelEconomy.gov. "How Vehicles Are Tested," "Detailed Test Information," "Why the Harmonic Mean is Used," "Factors That Affect Fuel Economy."
4. EPA/DOE FuelEconomy.gov. "Driving More Efficiently," "Fuel Economy in Cold Weather," "Keeping Your Car in Shape." Quantified factor impact ranges.
5. EPA 40 CFR 600.116. MPGe calculation methodology: 33.7 kWh per gallon gasoline equivalent.