Fuel economy is a measure of how far a car can travel on a given amount of fuel. In the United States, it’s expressed as miles per gallon (MPG), meaning the number of miles you can drive on one gallon of gasoline. The higher the number, the less fuel you use and the less you spend at the pump. The average new vehicle sold in the U.S. for model year 2024 hit a record high of 27.2 MPG.
MPG, L/100km, and MPGe
Different countries measure fuel economy in different ways. The U.S. uses miles per gallon, while most of Europe, Canada, and Australia use liters per 100 kilometers (L/100km). The key difference is that MPG measures distance per unit of fuel, while L/100km measures fuel per unit of distance. With MPG, a higher number is better. With L/100km, a lower number is better.
Some engineers at MIT have argued that the American system can actually be misleading. Thinking in “gallons per mile” gives you a clearer picture of how much fuel you’re actually burning, because the relationship between MPG and fuel saved isn’t straightforward. Going from 10 MPG to 20 MPG saves far more fuel than going from 30 MPG to 40 MPG, even though both are a 10 MPG jump. That’s why some advocates have pushed for a “gallons per 100 miles” label alongside the standard MPG rating.
For electric vehicles, the EPA created a unit called MPGe, or miles per gallon equivalent. It’s based on the fact that one gallon of gasoline contains the same energy as 33.7 kilowatt-hours of electricity. So an EV that can travel 100 miles on 33.7 kWh of electricity rates 100 MPGe. This lets you compare electric and gas-powered cars on a common scale, though it doesn’t directly translate to your electricity bill.
How the EPA Tests Fuel Economy
The MPG numbers on a new car’s window sticker come from EPA laboratory testing. Cars are placed on a dynamometer (essentially a treadmill for vehicles) and run through five standardized driving cycles designed to simulate real-world conditions:
- City cycle: Mimics stop-and-go rush-hour traffic at an average speed of 21 MPH, covering 11 miles over 31 minutes with 23 stops. About 18% of this test is spent idling.
- Highway cycle: Simulates steady highway driving at an average speed of 48 MPH over 10.3 miles, with gentle acceleration and no stops.
- High-speed/aggressive cycle: Tests harder acceleration and faster speeds.
- Hot weather test: Run at 95°F with the air conditioning cooling the cabin.
- Cold weather test: Run at 20°F with the heater and defrost on.
The city and highway cycles were originally developed decades ago, and they don’t perfectly reflect how most people actually drive. The highway test’s 48 MPH average, for instance, dates back to the era of the national 55 MPH speed limit. The three additional cycles were added later to make the final numbers more realistic. Still, your real-world MPG will vary depending on your driving style, climate, and routes.
Why City and Highway Numbers Differ
You’ll always see two MPG ratings on a car’s sticker: city and highway. Highway driving almost always produces better fuel economy in conventional gas cars because the engine operates more efficiently at steady, moderate speeds. City driving forces the engine through constant acceleration and braking cycles, and every time you accelerate from a stop, the engine burns extra fuel to overcome the car’s inertia. Idling at red lights burns fuel while covering zero miles, which drags down your average.
A compact sedan with a 2.0-liter engine burns roughly 0.16 gallons per hour just sitting at idle with no accessories running. A large sedan with a 4.6-liter engine burns about 0.39 gallons per hour. That fuel use adds up quickly in heavy traffic. Hybrids flip this equation somewhat because they can shut off the engine at idle and recapture energy during braking, which is why some hybrids actually get better city MPG than highway.
What Affects Your Car’s Fuel Economy
Several forces work against your car as it moves, and your engine has to burn fuel to overcome all of them.
Aerodynamic drag is the resistance your car faces pushing through air. At highway speeds around 60 MPH and above, air resistance is typically the single biggest drain on fuel. It increases dramatically with speed, which is why driving 80 MPH uses noticeably more fuel than driving 65. Reducing aerodynamic drag by even 10% can improve fuel efficiency by roughly the same percentage.
Vehicle weight matters most in city driving and on hills. Doubling a vehicle’s weight doubles its rolling resistance, meaning the engine works twice as hard just to keep the car moving. At lower speeds, rolling resistance from the tires dominates fuel consumption. At higher speeds, aerodynamic drag takes over, with the crossover point falling somewhere between 30 and 60 MPH depending on the vehicle.
Engine efficiency describes how much of the fuel’s energy actually becomes forward motion versus heat lost through the exhaust and cooling system. Modern gasoline engines convert only about 10% to 30% of their fuel’s energy into useful power, depending on conditions. Engines are least efficient at low power levels, like when you’re crawling through a parking lot, and most efficient when they’re working at a moderate, steady load.
Drivetrain losses account for energy lost between the engine and the wheels. The transmission and differential each lose about 3% of the engine’s power. Automatic transmissions lose additional energy through the torque converter. At low city speeds, drivetrain losses can eat up about 25% of total power. At highway speeds, that percentage drops to around 13% because the engine is producing more total power while the mechanical losses stay relatively constant.
How Maintenance and Tires Affect MPG
Tire pressure is one of the simplest things that can quietly erode your fuel economy. According to Department of Energy testing, driving with all four tires at 75% of the recommended pressure costs you 2 to 3% in fuel economy across all speeds. Let them drop to 50% of recommended pressure and the penalty jumps to around 5 to 10%, depending on how fast you’re driving. The loss is more severe at lower speeds, where rolling resistance plays a bigger role.
Radial tires produce up to 20% less rolling resistance than older bias-belted designs, which is one reason they’ve become the industry standard. Beyond tire type and pressure, keeping your engine properly maintained, using the recommended oil viscosity, and replacing clogged air filters all help your car deliver the fuel economy it was designed for. None of these fixes will transform your car’s efficiency, but together they prevent the gradual decline that makes you wonder why your tank doesn’t last as long as it used to.
Driving Habits That Move the Needle
Your right foot has more influence on fuel economy than almost any single mechanical factor. Aggressive acceleration forces the engine into its least efficient operating range, burning fuel at a much higher rate. Hard braking wastes the kinetic energy your engine just paid for in fuel. Smooth, gradual acceleration and coasting to decelerate keep the engine in a more efficient zone and reduce the total energy your car needs to complete a trip.
Speed matters too. Because aerodynamic drag increases with the square of your speed, driving at 80 MPH requires substantially more fuel per mile than driving at 60. Reducing unnecessary idling helps as well. If you’re going to be stopped for more than about 30 seconds, turning the engine off saves more fuel than restarting costs, which is exactly the logic behind the auto start-stop systems now common in new cars.