How to compare fuel economy correctly
The main source of confusion is that MPG, km/L, and L/100km describe the same thing in different ways. MPG and km/L go up when efficiency improves, while L/100km goes down.
If you are planning a trip or comparing vehicle ownership costs, these tools can help next: Trip Cost Calculator, Fuel Cost Comparison Calculator, and Car TCO Calculator.
MPG vs km/L vs L/100km
MPG means miles per gallon, km/L means kilometers per liter, and L/100km means liters used to travel 100 kilometers. They all describe fuel economy, but they are not equally intuitive. Many people find L/100km easier for estimating real fuel cost because it shows fuel used over a fixed distance.
Convert MPG to L/100km
Example: 40 MPG becomes 5.88 L/100km.
Convert MPG to km/L
Example: 40 MPG becomes 17.01 km/L.
US MPG and UK MPG are not the same
Always check whether the source uses a US gallon or an Imperial gallon. The UK gallon is larger, so UK MPG figures look higher for the same real-world fuel use. A UK review and a US window sticker can show different MPG values even when they refer to similar efficiency.
Quick rule
40 MPG (US) is 17.01 km/L, while 40 MPG (UK) is about 14.16 km/L. Confirm the standard before comparing two cars.
How to estimate fuel cost manually
Once you know the fuel economy, estimating trip cost is straightforward. Multiply fuel used over the distance by the local fuel price.
Using MPG
Example: 200 miles at 25 MPG uses 8 gallons. At $3.50 per gallon, the trip costs $28.00.
Using L/100km
Example: 300 km at 8 L/100km uses 24 liters. At €1.60 per liter, the trip costs €38.40.
Popular fuel economy conversions
These reference values cover the conversions people check most often when comparing cars across regions.
| MPG (US) | MPG (UK) | L/100km | km/L |
|---|---|---|---|
| 20 | 24.0 | 11.76 | 8.50 |
| 25 | 30.0 | 9.41 | 10.63 |
| 30 | 36.0 | 7.84 | 12.75 |
| 35 | 42.0 | 6.72 | 14.88 |
| 40 | 48.0 | 5.88 | 17.01 |
| 45 | 54.0 | 5.23 | 19.13 |
| 50 | 60.0 | 4.70 | 21.26 |
Frequently Asked Questions
- How do I convert MPG to km/L?
- To convert US MPG to km/L, multiply by 0.425144. Example: 40 MPG × 0.425144 = 17.01 km/L. For Imperial (UK) MPG, multiply by 0.354006. Always confirm whether your MPG figure is US or UK, because UK MPG is about 20% higher.
- How do I convert MPG to L/100km?
- To convert US MPG to L/100km, divide 235.215 by the MPG rating. For example, 30 MPG converts to 7.84 L/100km (235.215 ÷ 30). This formula accounts for the reciprocal nature of the units: MPG measures distance per volume, while L/100km measures volume per distance.
- How do I calculate fuel cost for a trip?
- Use the formula: (Distance ÷ MPG) × Fuel Price. For a 500-mile trip in a 30 MPG car with gas at $3.50/gal: (500 ÷ 30) × 3.50 = $58.33. Using metric: (Distance × L/100km ÷ 100) × Price/L. For 800km at 8L/100km and €1.80/L: (800 × 8 ÷ 100) × 1.80 = €115.20.
- What is considered good fuel economy?
- There is no single threshold because good fuel economy depends on vehicle size, drivetrain, and how the car is used. As a rough guide, 40 MPG (US) or better is strong for many gasoline passenger cars, while larger SUVs and trucks usually return lower figures.
- Why is UK MPG higher than US MPG?
- The UK Imperial gallon (4.546 L) is approximately 20% larger than the US liquid gallon (3.785 L). Therefore, a vehicle traveling 30 miles on a UK gallon is traveling further per unit of fuel than one on a US gallon. Always check the origin of the MPG figure; 30 MPG (US) equals about 36 MPG (UK).
- How can I save money on fuel?
- Drive smoothly, avoid unnecessary idling, keep tires properly inflated, remove excess cargo, and stay on top of maintenance. Small improvements in driving style and tire pressure can make a noticeable difference over time.
- What is MPGe and how does it relate to MPG?
- MPGe (Miles Per Gallon equivalent) is a metric used by the EPA to compare electric vehicles (EVs) with gas cars. It assumes 33.7 kWh of electricity contains the same energy as one gallon of gasoline. A rating of 100 MPGe indicates the EV is as efficient as a gas car getting 100 MPG.
- Why is L/100km arguably a better metric than MPG?
- L/100km is linear related to fuel consumption and cost. Saving 1 L/100km always saves the same amount of fuel over a fixed distance. MPG is non-linear: improving from 10 to 20 MPG saves far more fuel than improving from 40 to 50 MPG. This makes L/100km more intuitive for calculating actual savings.
- What affects my car's fuel efficiency the most?
- The biggest factors you can usually control are driving style, tire pressure, maintenance, excess weight, and aerodynamic drag at higher speeds. Weather, traffic, terrain, and short cold-start trips also affect real-world fuel economy.
- Does using air conditioning reduce fuel economy?
- It can. Air conditioning adds load to the engine or battery system, so fuel economy may drop, especially in slow traffic and hot weather. At highway speeds, open windows can also increase drag, so the more efficient choice depends on the situation.
- How do I convert kWh/100km to MPGe?
- Use the formula: MPGe = 2094.3 ÷ kWh/100km. For example, an EV consuming 15 kWh/100km is equivalent to approx. 140 MPGe (2094.3 ÷ 15). This helps compare EV efficiency directly with gas car MPG.
- What does 10 liters per 100 kilometers mean?
- It means your vehicle consumes 10 liters of fuel to travel a distance of 100 kilometers. In US terms, this equals roughly 23.5 MPG. Lower numbers (like 5.9 L/100km) represent better fuel economy, while higher numbers (like 11.8 L/100km) indicate higher fuel consumption.
- How do I convert 40 MPG to L/100km?
- 40 MPG (US) equals 5.88 L/100km. Use the formula 235.215 ÷ 40 to verify the result manually.
- What is 40 MPG in km/L?
- 40 MPG (US) is approximately 17.01 km/L. Calculation: 40 × 0.425144 = 17.01. If your figure is 40 MPG (UK), the result is about 14.16 km/L because Imperial gallons are larger.
- What is 30 MPG in L/100km?
- 30 MPG (US) converts to 7.84 L/100km. The manual calculation is 235.215 ÷ 30.