Gas Mileage Calculator: Fuel Economy & Cost per Mile
Short answer
This gas mileage calculator determines your vehicle's fuel economy in both miles per gallon (MPG) and liters per 100 kilometers (L/100 km), alongside your direct fuel cost per mile and overall trip expense. For example, driving 350 miles on 12 gallons of gas at $3.50 per gallon yields a fuel economy of 29.17 MPG (8.06 L/100 km), costing $0.12 per mile and $42.00 for the trip.
Formula
MPG = Miles Driven ÷ Gallons Used; L/100 km = 235.215 ÷ MPG; Cost per Mile = Gas Price ÷ MPG; Trip Cost = Gallons Used × Gas Price
MPG represents miles traveled per liquid gallon of fuel. L/100 km represents fuel consumption in liters per 100 kilometers, where 235.215 is the standard international conversion factor between US MPG and L/100 km. Gas Price is the cost per gallon in dollars, used to compute direct operating costs per mile and total trip fuel expenditure.
Worked example
- Miles driven
- 350
- Gallons used
- 12
- Gas price
- 3.50
- Fuel economy: 29.17 MPG
- Liters per 100 km: 8.06
- Cost per mile: 0.12
- Trip fuel cost: 42.00
What the Gas Mileage Calculator Measures
The Gas Mileage Calculator evaluates the fuel performance of any internal combustion or hybrid vehicle. Whether you are budgeting for a cross-country road trip, tracking real-world fuel consumption against manufacturer window stickers, or splitting fuel costs with a passenger, this tool provides four critical figures:
- Fuel Economy (MPG): The number of miles traveled for each gallon of gasoline consumed.
- Fuel Consumption (L/100 km): The metric standard indicating how many liters of fuel are burned to travel 100 kilometers.
- Cost per Mile: The direct fuel expense incurred for every mile driven.
- Trip Fuel Cost: The total dollar amount spent on gasoline for the recorded distance.
This tool is designed for daily commuters, fleet managers, road-trip planners, and vehicle owners looking to monitor their engine health through ongoing efficiency tracking.
Step-by-Step Worked Example
To see how the underlying math works, consider a driver who travels 350 miles, consumes 12 gallons of regular unleaded fuel, and pays $3.50 per gallon.
1. Calculate Fuel Economy (MPG)
Divide the total distance traveled by the volume of fuel required: $$\text{MPG} = \frac{\text{Miles}}{\text{Gallons}} = \frac{350}{12} = 29.1667\dots \approx 29.17\text{ MPG}$$
2. Convert to Liters per 100 Kilometers (L/100 km)
Using the standard conversion constant for US liquid gallons and statute miles ($235.215$): $$\text{L/100 km} = \frac{235.215}{\text{MPG}} = \frac{235.215}{29.1667} \approx 8.06\text{ L/100 km}$$
3. Calculate Cost per Mile
Divide the price per gallon by your calculated MPG: $$\text{Cost per Mile} = \frac{\text{Gas Price}}{\text{MPG}} = \frac{$3.50}{29.1667} \approx $0.12\text{ per mile}$$
4. Calculate Total Trip Fuel Cost
Multiply the total gallons consumed by the fuel price per gallon: $$\text{Trip Cost} = \text{Gallons} \times \text{Gas Price} = 12 \times $3.50 = $42.00$$
How to Interpret Your Fuel Economy Results
Fuel economy numbers vary widely by vehicle classification, powertrain design, and driving environment:
- Under 20 MPG (Above 11.75 L/100 km): Common for full-size pickups, heavy-duty commercial vans, large three-row SUVs, and high-displacement performance sports cars.
- 20 to 29 MPG (8.11 to 11.75 L/100 km): Typical for standard mid-size crossover SUVs, mid-size sedans with turbocharged four-cylinder engines, and light-duty trucks under mixed driving conditions.
- 30 to 39 MPG (6.03 to 7.84 L/100 km): Standard benchmark for modern compact sedans, subcompact hatchbacks, and highly efficient highway cruisers.
- 40+ MPG (Below 5.88 L/100 km): Standard territory for traditional hybrid electric vehicles (HEVs) and specialized diesel passenger cars.
A sudden drop of 10% to 15% in your regular MPG over consecutive fill-ups often signals a mechanical issue, such as an underinflated tire, a failing oxygen sensor, a dragging brake caliper, or a clogged engine air filter.
Practical Tips for Improving Gas Mileage
- Maintain Recommended Tire Pressure: Running tires 5 to 10 PSI below the vehicle placard pressure increases rolling resistance and reduces fuel economy by 1% to 3%.
- Avoid Aggressive Acceleration and Braking: Rapid acceleration and hard braking consume significantly more fuel than smooth, gradual throttle inputs.
- Use Cruise Control on Flat Highways: Maintaining a steady speed eliminates micro-accelerations that burn unnecessary fuel.
- Remove Unneeded Cargo: Carrying an extra 100 pounds in your trunk or rear cargo area can reduce fuel efficiency by roughly 1% in smaller vehicles.
- Limit Extended Idling: An idling car achieves exactly 0.00 MPG. Modern fuel-injected engines use less fuel restarting than idling for more than 10 to 15 seconds.
When Not to Rely Exclusively on This Calculator
While this calculator provides exact mathematical answers for your inputs, certain variables affect its real-world application:
- Partial Fill-Ups: If you do not fill your fuel tank completely at both the start and end of your measurement period, the gallons entered will not match the miles traveled, leading to inaccurate MPG calculations.
- Aftermarket Wheel and Tire Sizes: Changing your tire diameter without recalibrating your speedometer causes your odometer to over- or under-report total mileage.
- Plug-in Hybrid (PHEV) Usage: If a vehicle draws energy from both an external electrical outlet and liquid gasoline, standard MPG formulas fail to capture the true total energy cost without factoring in kilowatt-hours (kWh).
Comparison
| Scenario | Input | Result |
|---|---|---|
| Lower input | Miles driven: 280 | Fuel economy: 23.33 MPG |
| Base example | Miles driven: 350 | Fuel economy: 29.17 MPG |
| Higher input | Miles driven: 420 | Fuel economy: 35 MPG |
Common mistakes
- Using tank capacity instead of actual gallons pumped to refill the tank
- Estimating fuel consumption from partial fill-ups rather than a full-to-full measurement
- Relying solely on dashboard trip computers, which often read 2% to 5% higher than actual fuel economy
- Comparing imperial gallons (UK) with US liquid gallons without standardizing units
- Ignoring tire pressure variations, which can skew both odometer accuracy and actual fuel efficiency
Checklist
- Fill your tank completely until the pump clicks off automatically
- Reset your trip odometer to 0 or record your starting mileage
- Drive normally until your next fuel fill-up is required
- Fill the tank completely again at the same pump or pump shut-off setting
- Record the exact gallons pumped from the fuel receipt
- Record the exact miles driven shown on the trip odometer
- Note the price per gallon paid to evaluate total and per-mile costs
Calculators Percentage Editorial Team
Calculator guides from the Calculators Percentage team. These introductory examples are educational, not professional advice.
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This calculator computes direct fuel expenses based on user-entered values. It does not account for indirect operational costs such as engine oil, maintenance, depreciation, insurance, or tire wear.