How MPG Is Calculated
Miles per gallon (MPG) measures how far a vehicle travels on one gallon of fuel. The formula is straightforward: divide the distance traveled by the fuel consumed.
For example, if you drove 350 miles and used 14 gallons of gas, your fuel economy is 350 / 14 = 25 MPG.
The most accurate way to measure MPG is the tank-to-tank method: fill up completely, reset your trip odometer, drive normally, then fill up again. The gallons at the second fill-up divided into the trip miles gives your real-world MPG.
Quick Reference
MPG vs L/100km
Most countries outside the US use liters per 100 kilometers (L/100km) to measure fuel efficiency. The two are inversely related:
| MPG | L/100km | Rating |
|---|---|---|
| 15 | 15.7 | Poor |
| 20 | 11.8 | Below Average |
| 25 | 9.4 | Average |
| 30 | 7.8 | Good |
| 35 | 6.7 | Very Good |
| 40 | 5.9 | Excellent |
| 50 | 4.7 | Hybrid Territory |
How to Improve Fuel Economy
Driving habits have the biggest impact on real-world MPG. Aggressive acceleration and braking can reduce fuel economy by 15-30% in city driving and 10-20% on the highway.
The most effective ways to improve MPG:
- Maintain steady speed - use cruise control on highways. Each 5 MPH over 50 costs you roughly 7% more fuel.
- Check tire pressure monthly - under-inflated tires increase rolling resistance. Even 5 PSI low costs 1-2% MPG.
- Remove excess weight - every 100 lbs of extra cargo reduces MPG by about 1% in a midsize car.
- Reduce idling - idling gets 0 MPG. Turn off the engine if stopped for more than 60 seconds.
- Use the recommended oil - the right viscosity reduces internal engine friction.
- Keep up with maintenance - a dirty air filter, worn spark plugs, or bad O2 sensor can reduce MPG by 5-20%.
Easiest MPG Improvement
Fuel Cost Math
To calculate your cost per mile, divide the gas price by your MPG. At $3.50/gallon and 25 MPG, each mile costs $3.50 / 25 = $0.14.
The average American drives 12,000 miles per year. At 25 MPG and $3.50/gallon, that is 12,000 / 25 × $3.50 = $1,680 per year in fuel. Improving to 35 MPG drops that to $1,200 - saving $480 annually.
City vs Highway MPG
EPA vs Real-World MPG
The EPA tests vehicles on a dynamometer in a lab, not on real roads. The combined MPG rating uses a weighted average: 55% city / 45% highway. In practice, most drivers achieve 10-20% worse than the EPA combined number.
The gap is larger for vehicles with high horsepower (temptation to accelerate harder) and for city-heavy commuters (stop-and-go traffic is harder on fuel economy than EPA city testing simulates).
fueleconomy.gov publishes user-reported real-world MPG data alongside EPA estimates for most vehicles. This crowd-sourced data is often the best predictor of what you will actually see.
EPA Ratings vs Reality
Factors That Affect MPG
Temperature has a major effect. Cold weather (20°F vs 77°F) can reduce fuel economy by 15-25% on short trips due to cold engine friction, slower warm-up, and denser air. In summer, AC use costs 3-5% MPG in city driving.
Terrain matters too. Mountainous driving consumes more fuel climbing hills than you recover descending. Wind resistance increases fuel consumption - a 20 MPH headwind at 60 MPH can reduce MPG by 10-15%.
Vehicle load directly affects fuel economy. Roof racks increase aerodynamic drag by 2-8% even when empty. A fully loaded roof box can reduce highway MPG by 10-25%. Remove cargo carriers when not in use.
Frequently Asked Questions
Fill up your tank completely, reset your trip odometer, drive normally until you need to refuel, then fill up again. Divide the miles driven by the gallons used at the second fill-up. For example, 300 miles divided by 12 gallons equals 25 MPG.
The average new car in the US gets about 28 MPG combined (city + highway). Above 30 MPG is good, above 35 MPG is very good, and above 40 MPG is excellent. Hybrids typically achieve 45-60 MPG. For trucks and SUVs, 22-28 MPG is considered good.
Divide 235.215 by your MPG value. For example, 30 MPG = 235.215 / 30 = 7.84 L/100km. The relationship is inverse - lower L/100km means better efficiency. A car getting 7 L/100km is more efficient than one getting 10 L/100km.
EPA tests are conducted in a lab under controlled conditions. Real-world driving includes aggressive acceleration, idling in traffic, cold starts, AC use, and wind. Most drivers see 10-20% lower MPG than the EPA combined estimate. Highway MPG is typically closer to the rating than city MPG.
Yes, significantly. Most vehicles achieve peak fuel economy between 35-55 MPH. Above 55 MPH, aerodynamic drag increases exponentially - every 5 MPH above 50 is roughly equivalent to paying an extra $0.25 per gallon. At 70 MPH, fuel economy is typically 15-25% worse than at 55 MPH.
Under-inflated tires increase rolling resistance and reduce fuel economy by about 0.2% for every 1 PSI drop below the recommended pressure. If all four tires are 10 PSI low, you could be losing 3-5% fuel economy, which adds up to $100-200 per year in wasted fuel at average US driving levels.