The Quick Answer
Fuel economy measures how far your vehicle travels per unit of fuel. The two main units:
- MPG (US): Miles Per Gallon = Distance (miles) ÷ Fuel used (gallons)
- L/100km: Liters per 100 Kilometers = (Fuel used in liters × 100) ÷ Distance (km)
A car that drives 300 miles on 10 gallons of gas gets 30 MPG, which equals 7.8 L/100km.
To convert between them: L/100km = 235.215 ÷ MPG (and vice versa).
How to Measure Your Actual Fuel Economy
The EPA rating on your car's window sticker is an estimate. Your real-world fuel economy depends on how and where you drive. Here's how to measure it accurately.
The Fill-to-Fill Method
This is the standard technique used by automotive engineers and enthusiasts:
- Fill your tank completely at a gas station. Stop at the first automatic click.
- Reset your trip odometer to zero (or note the current reading).
- Drive normally until you need fuel again.
- Fill up again at the same pump if possible. Stop at the first click again.
- Record two numbers: miles driven (from the trip odometer) and gallons added (from the pump receipt).
Now calculate:
MPG = Miles driven ÷ Gallons added
Example: You drove 287 miles and added 9.8 gallons. 287 ÷ 9.8 = 29.3 MPG
Why "Fill to Fill" Works
The key insight is that the second fill-up replaces exactly the fuel you consumed. By filling to the same level both times (the auto-shutoff point), the gallons on your receipt represent the fuel burned during those miles.
Getting More Accurate Numbers
A single tank can be misleading. Conditions vary — a highway road trip will give very different numbers than a week of city commuting. For a reliable average:
- Track 3–5 consecutive fill-ups and average the results
- Keep conditions consistent (same type of driving, same season)
- Always stop at the first click — topping off adds error
- Use the same pump if practical (pump auto-shutoffs vary slightly)
Many drivers use a simple spreadsheet: date, odometer reading, gallons, and a calculated MPG column.
MPG vs L/100km: Two Ways to Say the Same Thing
Different countries use different units, and they measure the concept in opposite directions:
| Unit | Measures | Higher number means | Used in |
|---|---|---|---|
| MPG (US) | Distance per fuel | More efficient | United States |
| MPG (UK/Imperial) | Distance per fuel | More efficient | United Kingdom |
| L/100km | Fuel per distance | Less efficient | Europe, Canada, Australia |
| km/L | Distance per fuel | More efficient | Japan, India, parts of Asia |
The US gallon (3.785 liters) is smaller than the Imperial gallon (4.546 liters), so UK MPG numbers are always higher than US MPG for the same car.
Conversion Formulas
- US MPG → L/100km: Divide 235.215 by the MPG value
- L/100km → US MPG: Divide 235.215 by the L/100km value
- US MPG → km/L: Multiply MPG by 0.4251
- UK MPG → US MPG: Multiply by 0.8327
Worked example: A European car rated at 5.5 L/100km: 235.215 ÷ 5.5 = 42.8 MPG (US)
What Fuel Economy Numbers Actually Look Like
Real-world fuel economy varies widely by vehicle type. These ranges reflect typical 2020s vehicles under mixed driving conditions:
| Vehicle type | Typical US MPG | Typical L/100km |
|---|---|---|
| Hybrid sedan | 45–58 | 4.1–5.2 |
| Compact car | 30–40 | 5.9–7.8 |
| Midsize sedan | 25–35 | 6.7–9.4 |
| Small SUV / crossover | 25–32 | 7.4–9.4 |
| Large SUV | 18–25 | 9.4–13.1 |
| Pickup truck | 15–23 | 10.2–15.7 |
| Sports car | 18–28 | 8.4–13.1 |
| Electric vehicle (MPGe) | 90–130+ | N/A (uses kWh) |
Note: Electric vehicles use a different metric called MPGe (miles per gallon equivalent), which converts electricity consumption to a gasoline-equivalent figure for comparison purposes.
The 7 Biggest Factors That Affect Fuel Economy
1. Driving Speed
Aerodynamic drag increases with the square of speed. Most vehicles reach peak fuel efficiency between 35–50 mph (55–80 km/h). Above 50 mph, every 5 mph increase costs roughly 7–14% more fuel, depending on the vehicle's aerodynamic profile.
2. Acceleration and Braking Patterns
Aggressive acceleration followed by hard braking wastes fuel. Smooth, gradual acceleration and coasting to decelerate can improve fuel economy by 15–30% in city driving compared to an aggressive style.
3. City vs Highway Driving
City driving involves frequent stops, starts, and idling — all fuel-intensive. Highway driving is more efficient because the engine operates near its optimal load. The EPA rates vehicles for both: a car might get 28 MPG city and 36 MPG highway.
4. Vehicle Weight and Load
Every additional 100 pounds (45 kg) reduces fuel economy by roughly 1–2%. Roof racks, cargo boxes, and heavy loads in the trunk all add up. An empty roof rack alone can reduce highway MPG by 2–5% due to added aerodynamic drag.
5. Tire Pressure
Under-inflated tires increase rolling resistance. Each 1 PSI drop below the recommended pressure reduces fuel economy by about 0.2%. Tires that are 8 PSI low (common in cold weather) can cost you 1.5% or more.
6. Temperature and Weather
Cold engines are less efficient. Short trips in winter — where the engine never fully warms up — can reduce fuel economy by 12–20% compared to the same trips in warm weather. Air conditioning in summer typically costs 3–10% depending on conditions.
7. Maintenance
A clogged air filter, old spark plugs, or misaligned wheels all hurt efficiency. Keeping up with scheduled maintenance is the simplest way to maintain the fuel economy your vehicle was designed to deliver.
Common Mistakes When Measuring Fuel Economy
Relying on the car's trip computer
Most in-dash MPG displays are optimistic by 5–15%. They calculate fuel consumption using injector pulse width estimates, which drift over time. The fill-to-fill method with actual pump readings is more accurate.
Comparing MPG numbers linearly
MPG is not a linear scale. Improving from 15 to 20 MPG saves far more fuel than improving from 35 to 40 MPG — even though both are "5 MPG better."
Over 10,000 miles:
- 15 MPG → 20 MPG: saves 167 gallons (667 → 500)
- 35 MPG → 40 MPG: saves 36 gallons (286 → 250)
This is why L/100km is arguably a better metric for comparing efficiency improvements — the scale is linear.
Not accounting for ethanol content
In many countries, gasoline contains 10% ethanol (E10) or more. Ethanol has about 27% less energy per gallon than pure gasoline. If you switch between E10 and E15 or E85 fuel, your MPG will change noticeably, but it doesn't mean your car's efficiency changed.
Measuring over too short a distance
Calculating MPG over a single short trip (say, 50 miles) produces noisy results. Wind, elevation changes, traffic lights, and even which direction you drove can dominate the measurement. Use at least a full tank — preferably several — for a reliable number.
How to Estimate Fuel Cost for a Trip
Once you know your fuel economy, estimating trip cost is straightforward:
Fuel cost = (Distance ÷ MPG) × Fuel price per gallon
Example: A 450-mile road trip in a car that gets 28 MPG, with gas at $3.40/gallon:
450 ÷ 28 = 16.07 gallons 16.07 × $3.40 = $54.64
For metric: Fuel cost = (Distance in km ÷ 100) × L/100km × Price per liter
Example: An 800 km trip at 7.2 L/100km with fuel at €1.65/liter:
(800 ÷ 100) × 7.2 = 57.6 liters 57.6 × €1.65 = €95.04
Use the fuel economy calculator to run these numbers quickly, or the gas cost calculator for trip-specific estimates.
EPA Ratings vs Real-World Performance
The EPA tests vehicles on a dynamometer (a treadmill for cars) using standardized drive cycles. The test conditions include:
- Controlled temperature (68–86°F / 20–30°C)
- No wind or elevation changes
- Standardized acceleration patterns
- Air conditioning off for the base test
Real-world results typically come in 10–20% below EPA ratings. Highway numbers tend to be closer to the rating; city numbers show more variance because individual driving styles differ greatly in stop-and-go conditions.
The EPA adjusted its methodology in 2008 to be more realistic, but the gap still exists. Treat EPA numbers as a useful comparison tool between vehicles, not as a guarantee of what you'll achieve.
FAQ
How do I calculate miles per gallon?
Divide the distance driven (in miles) by the fuel used (in gallons). For example, 350 miles on 12.5 gallons = 28 MPG.
What is a good MPG?
It depends on the vehicle type. For a passenger car, 30+ MPG is good and 40+ is excellent. SUVs and trucks have lower baselines — 25 MPG is respectable for a midsize SUV.
How do I convert MPG to L/100km?
Divide 235.215 by the MPG value. For example, 30 MPG = 235.215 ÷ 30 = 7.84 L/100km.
How do I convert L/100km to MPG?
Divide 235.215 by the L/100km value. For example, 6.5 L/100km = 235.215 ÷ 6.5 = 36.2 MPG.
Why is my actual MPG lower than the EPA estimate?
EPA tests use controlled conditions (moderate temperature, no wind, standardized driving). Real-world factors like aggressive driving, cold weather, short trips, AC use, extra weight, and hilly terrain all reduce efficiency.
Does cruise control save fuel?
Generally yes, especially on flat highways. It prevents the small speed fluctuations that waste fuel. However, on hilly terrain, cruise control can be less efficient because it accelerates hard to maintain speed uphill.
Does AC use more fuel than open windows?
At low speeds (under ~45 mph), open windows are more efficient. At highway speeds, the aerodynamic drag from open windows can cost more fuel than the AC compressor uses. The crossover point varies by vehicle.
How much fuel does idling use?
A typical passenger car burns 0.2–0.5 gallons per hour at idle (about 0.8–1.9 liters per hour). If you'll be stopped for more than 30–60 seconds, turning the engine off saves fuel — which is why many modern cars have auto-stop systems.
Does premium fuel give better MPG?
Only if your engine is designed for it. Engines with higher compression ratios that require premium will get slightly better economy on the correct fuel. Putting premium in a car designed for regular fuel provides no benefit.
Is MPG or L/100km a better measurement?
L/100km is mathematically more useful for comparing efficiency improvements because it's a linear scale. MPG is an inverse relationship — the same numerical improvement means different things at different points on the scale. Both convey the same underlying information.
How do electric car MPGe numbers work?
MPGe (miles per gallon equivalent) converts electricity consumption to a gasoline-equivalent figure. The EPA defines 33.7 kWh of electricity as equivalent to one gallon of gasoline (based on energy content). An EV rated at 100 MPGe uses 33.7 kWh to travel 100 miles.
Does tire size affect fuel economy?
Yes. Larger, wider tires increase rolling resistance and weight, which reduces fuel economy. The difference is typically 1–3% for common tire size changes. Keeping any tire at its recommended pressure matters more than the size itself.
Try It Yourself
Use the fuel economy calculator to calculate your vehicle's MPG or L/100km from your last fill-up. Enter the distance driven and fuel used, and optionally add the fuel price to see your cost per mile or cost per 100 km.