How to use the fuel efficiency calculator
- Choose your currency and units: km and liters, miles and US gallons, or miles and liters (UK).
- Under Calculate, pick Fuel efficiency to find your car's MPG or L/100km, or Trip cost to find what a drive will cost.
- Enter the trip distance and the fuel used (or your car's fuel efficiency), plus the fuel price if you want the cost.
- Press Calculate. Your choices are remembered in this browser for next time.
How to measure your real fuel efficiency
The figure on a car's window sticker is measured in a lab. Your own driving can be quite different, so the most accurate number comes from a full tank:
- Fill the tank until the pump clicks off, and reset your trip meter (or write down the odometer reading).
- Drive normally until the tank is at least half empty.
- Fill up again until the pump clicks off. The amount on the pump is the fuel you used.
- Enter the distance on the trip meter and the fuel from the pump into the calculator.
Repeating this over a few tanks and averaging the results smooths out differences in how full the pump fills the tank.
Fuel efficiency formulas
Fuel efficiency is either distance per unit of fuel (MPG, km/L), where higher is better, or fuel per distance (L/100km), where lower is better.
- MPG = miles driven ÷ gallons used
- km/L = kilometers driven ÷ liters used
- L/100km = liters used ÷ kilometers driven × 100
- Trip fuel cost = distance ÷ fuel efficiency × fuel price
Examples
- You drive 300 miles on 10 US gallons: 300 ÷ 10 = 30 mpg. At $3.50 a gallon, that trip cost 10 × $3.50 = $35.00, or about 11.7 cents a mile.
- You drive 450 km on 36 liters: 450 ÷ 36 = 12.5 km/L, which is 36 ÷ 450 × 100 = 8.0 L/100km.
Converting between MPG, L/100km and km/L
A US gallon is 3.785 liters and a UK (imperial) gallon is 4.546 liters, so UK MPG figures are about 20% higher than US MPG for the same car.
| From | To | Formula |
|---|---|---|
| MPG (US) | L/100km | 235.215 ÷ MPG |
| MPG (UK) | L/100km | 282.481 ÷ MPG |
| km/L | L/100km | 100 ÷ km/L |
| MPG (US) | km/L | MPG × 0.4251 |
| MPG (US) | MPG (UK) | MPG × 1.201 |
| MPG (US) | MPG (UK) | L/100km | km/L |
|---|---|---|---|
| 20 | 24.0 | 11.8 | 8.5 |
| 25 | 30.0 | 9.4 | 10.6 |
| 30 | 36.0 | 7.8 | 12.8 |
| 35 | 42.0 | 6.7 | 14.9 |
| 40 | 48.0 | 5.9 | 17.0 |
| 50 | 60.0 | 4.7 | 21.3 |
Ways to improve your fuel efficiency
- Drive smoothly. Hard acceleration and braking use much more fuel than gentle, steady driving.
- Slow down on the highway. Air resistance rises quickly with speed, so most cars use noticeably more fuel above about 50 mph (80 km/h).
- Keep your tires at the right pressure. Under-inflated tires roll less easily. The correct pressure is on a sticker inside the driver's door.
- Remove extra weight and roof racks. Heavy cargo and anything on the roof make the engine work harder.
- Don't idle for long. An idling engine uses fuel while going nowhere.
- Keep up with servicing. Clean air filters, the right oil and a well-tuned engine all help.
The US government's fueleconomy.gov has more detail on how driving habits affect fuel use.
Frequently asked questions
What is a good fuel efficiency?
It depends on the vehicle. Many modern small cars manage around 35 to 40 mpg (US), or 6 to 7 L/100km, while larger SUVs and pickups are often between 18 and 25 mpg (9.5 to 13 L/100km). Hybrids can do considerably better.
Is a higher or lower L/100km better?
Lower. L/100km is the fuel needed to drive 100 km, so a car using 5 L/100km is more efficient than one using 8 L/100km. With MPG and km/L, higher is better.
Why is my real MPG lower than the official figure?
Official figures come from standard lab tests. Short trips, cold weather, heavy traffic, high speeds, air conditioning and extra weight all lower real-world fuel efficiency.
What's the difference between US and UK MPG?
The gallon. A UK gallon is about 20% bigger than a US gallon, so the same car shows a higher MPG in the UK. Choose "miles and liters (UK)" in the calculator to get UK MPG.