Trip estimates

Gas Mileage Calculator

Enter miles driven, gallons used, gas price, and currency to calculate measured MPG, trip fuel cost, cost per mile, and liters per 100 km.

Last reviewed May 17, 2026 by ToolSpilo Editorial Team.

Review method: Measured-MPG math, full-tank workflow, and fuel-economy caveats reviewed against current transportation references.

For planning purposes only. Actual costs may vary. Not financial advice.

Calculator tool

How this calculator works

Use the explanation to understand the formula, assumptions, and practical limits behind the calculator result.

What Does the Gas Mileage Calculator Measure?

This calculator measures actual fuel economy from a completed drive or fill-up interval. It is different from a trip-planning calculator: here, the fuel used is already known, so the result tells you how efficiently the vehicle really performed.

MPG Formula

MPG=Miles drivenGallons usedMPG = \frac{\text{Miles driven}}{\text{Gallons used}}

The calculator also shows:

  • Trip fuel cost = gallons used × gas price
  • Cost per mile = trip fuel cost ÷ miles driven
  • L/100 km=235.214/MPGL/100\text{ km} = 235.214 / MPG

Worked Example

If a vehicle travels 300 miles and uses 10 gallons:

MPG=300/10=30MPG = 300 / 10 = 30

At 3.50 per gallon, the fuel cost is 35.00, and the cost per mile is about 0.117.

Best Way to Measure Real MPG

For a cleaner estimate, use the full-tank method:

  1. Fill the tank completely.
  2. Reset the trip odometer.
  3. Drive normally.
  4. Refill the tank to the same level.
  5. Divide the miles driven by the gallons added at refill.

A single tank can be noisy. A multi-tank average is usually more useful if you want to compare maintenance, driving habits, or seasonal changes.

Why MPG Changes

Actual fuel economy can move with cold weather, air conditioning, idling, tire pressure, load, route type, speed, and aggressive driving. That is why one tank should be treated as a measurement, not a permanent verdict on the vehicle.

Frequently asked questions

Why is the full-tank method better than guessing from the fuel gauge?

Because fuel gauges are not precise measuring tools. Filling to the same level before and after the trip lets the gallons added at the pump represent the fuel actually used during that interval.

Why is my measured MPG lower than the window-sticker number?

Sticker values are standardized estimates, while your result reflects your route, weather, speed, traffic, accessory use, and driving style. Treat the calculator as a measurement of your own use, not as a contradiction of a lab rating.

How many fill-ups should I average?

If you only need one trip result, one fill-up is enough. If you want a dependable view of the vehicle, average several comparable fill-ups so one unusual route, weather day, or partial refill does not dominate the answer.

Why does winter reduce my gas mileage?

Cold starts, denser air, lower tire pressure, winter fuel blends, and longer warm-up time all reduce efficiency. Short winter trips are usually affected more than longer drives.