Gas Mileage (MPG)

Calculating Your Vehicle’s Gas Mileage

Gas mileage (MPG) is the distance a vehicle travels divided by the amount of fuel it uses, expressed in miles per gallon. Enter the distance you traveled and how much fuel you used to cover it, and this calculator returns your fuel economy — in MPG, and as the metric equivalent (liters per 100 kilometers) used in most of the rest of the world.

To measure your own vehicle’s real-world mileage: fill the tank completely, reset your trip odometer (or note the current reading), drive normally until you need to refuel, then fill up again and note exactly how much fuel it took. That fuel amount and the distance since your last fill-up are exactly what this calculator needs.

The Formula

$$\text{MPG} = \frac{\vA{\text{Distance (miles)}}}{\vB{\text{Fuel Used (gallons)}}}$$ $$\text{L/100km} = \left(\frac{\vB{\text{Fuel Used (L)}}}{\vA{\text{Distance (km)}}}\right) \times 100$$

Note that these two figures move in opposite directions for the same fuel efficiency: a higher MPG is better (more distance per gallon), while a lower L/100km is better (less fuel needed per 100 kilometers) — they aren’t simply the same number converted, since one measures “distance per fixed fuel” and the other measures “fuel per fixed distance.”

Worked Example

A trip of 100 km using 8 liters of fuel:

1. Converted to imperial units: about $\vA{62.1}$ miles using about $\vB{2.11}$ gallons.
2. MPG: $\vA{62.1} \div \vB{2.11} \approx 29.4 \text{ MPG}$.
3. Metric equivalent: $(\vB{8} \div \vA{100}) \times 100 = 8.0 \text{ L per 100 km}$.

Key Factors to Consider

• A single fill-up’s calculation can be skewed by an incomplete fill or a pump that stops early on the previous fill-up. For an accurate measurement, always fill the tank completely both times (not just “topping off”) — a partial fill on either end throws off the fuel-used figure and therefore the whole calculation.
• Averaging several fill-ups gives a more reliable figure than a single tank. City vs. highway mix, weather, and driving style all vary from tank to tank — calculating your mileage over several fill-ups and averaging the results smooths out this natural variation better than trusting any single measurement.
• Tire pressure, cargo weight, and driving speed all measurably affect real-world fuel economy. Underinflated tires, excess cargo weight, and higher highway speeds all increase fuel consumption — if your calculated mileage seems lower than expected, these are common, checkable causes.
• Official EPA-style fuel economy ratings use standardized test conditions your own driving won’t exactly match. A window-sticker rating comes from a controlled test cycle — your actual calculated mileage from real-world driving is a genuinely different (and often more personally useful) number than the official rating, not a discrepancy to be alarmed by.

Common Mistakes

• Topping off the tank instead of filling it completely at both fill-ups. As covered above, this calculation only works correctly when both the starting and ending fill-ups are genuinely full — a partial “top off” understates or overstates the true fuel-used figure and throws off the whole result.
• Mixing up which unit goes in which field. Entering a distance in kilometers alongside a fuel amount in gallons (or miles alongside liters) produces a mileage figure that looks plausible but is actually meaningless — double-check that distance and fuel volume are both in the unit system you intend.
• Judging fuel economy from a single fill-up rather than several averaged together. As covered above, city/highway mix, weather, and driving style all vary from tank to tank — one fill-up can look unusually good or bad purely by chance, while an average across several tanks reflects your real typical mileage.
• Assuming a lower L/100km number means worse efficiency, the way a lower MPG does. As covered above, the two figures run in opposite directions — L/100km measures fuel needed per fixed distance, so a LOWER number there is actually MORE efficient, the reverse of how MPG works.

Useful to Know

• Once you know your vehicle’s real MPG or L/100km from this calculator, the Fuel Cost Calculator uses that figure (plus a distance and fuel price) to estimate what a specific trip will cost.
• The Fuel Savings Calculator compares this vehicle’s mileage against another vehicle’s to show how much switching would actually save — or cost — per year.
• A recent tire size change can shift a vehicle’s actual odometer accuracy slightly, which in turn affects a mileage calculation based on the trip odometer — the Tire Size Calculator shows how much a size change moves the odometer’s real-world accuracy.