How to calculate fuel cost
For example, a 300-mile trip in a car that gets 30 MPG uses 10 gallons; at $3.50 a gallon that's $35. In metric, fuel needed = distance × L/100 km ÷ 100.
How to calculate your gas mileage
- Fill the tank completely and reset your trip odometer (or note the odometer reading).
- Drive normally until you need fuel again.
- Fill up completely and note how many gallons (or liters) it took and the miles driven.
- Divide miles driven by gallons used. Averaging over several fill-ups gives the most accurate figure.
MPG vs. L/100 km
The US measures fuel economy as distance per unit of fuel (miles per gallon — higher is better), while most other countries use fuel per distance (liters per 100 km — lower is better). To convert: L/100 km = 235.21 ÷ MPG. This calculator shows both.
Ways to save on fuel
- Keep tires properly inflated and remove unnecessary weight and roof racks.
- Accelerate gently and keep to the speed limit — fuel economy usually drops sharply above about 50–60 mph.
- Use the vehicle comparison above before buying: a few MPG makes a big difference over years of driving.
Driving for work? Log your trips with the Mileage Calculator to work out your reimbursement.
Fuel cost by gas mileage
With gas at $3.50 a gallon:
| Fuel economy | Cost per 100 miles | Cost per year (12,000 miles) |
|---|---|---|
| 15 MPG | $23.33 | $2,800 |
| 20 MPG | $17.50 | $2,100 |
| 25 MPG | $14.00 | $1,680 |
| 30 MPG | $11.67 | $1,400 |
| 40 MPG | $8.75 | $1,050 |
| 50 MPG | $7.00 | $840 |
Notice that the savings shrink as mileage improves. Going from 15 to 20 MPG saves $700 a year, while going from 40 to 50 MPG saves only $210. Replacing a very inefficient vehicle with a moderately efficient one saves more fuel than upgrading an already efficient car — which is easier to see in gallons per mile than in miles per gallon.
Frequently asked questions
Why is my real mileage lower than the official rating?
Official figures come from standardized tests. Real driving includes short trips with a cold engine, traffic, hills, high speeds, air conditioning, cargo and winter fuel blends, all of which reduce economy. Measuring your own mileage over several tanks, as described above, gives the truest figure.
Does premium gas improve mileage?
Only if your engine is designed for it. If the owner's manual says premium is required, use it. If it says premium is merely recommended, or the car is designed for regular, you'll rarely see enough benefit to justify the price.
How should we split fuel costs on a road trip?
Enter the trip distance and choose round trip, then set "Split between" to the number of people. Many groups have passengers share the fuel and let the driver, who is providing the car and taking on the wear, pay a smaller share or nothing.
How do I compare a gas car with an electric car?
Compare the cost per mile. For the gas car, divide the fuel price by MPG: $3.50 ÷ 30 = about 11.7 cents a mile. For an electric car, multiply its consumption in kWh per mile by your electricity rate: 0.30 kWh × $0.17 = about 5.1 cents a mile. The Electricity Cost Calculator can help with charging costs.
Is a UK gallon the same as a US gallon?
No. An imperial gallon is about 20% larger (4.546 liters against 3.785), so a car's MPG figure in the UK is about 20% higher than the same car's US figure. The gas mileage tool above shows both.