Softonic

MPG Calculator

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Your dashboard MPG readout is an estimate. For an honest figure, fill the tank, zero the trip odometer, drive normally, then fill up again and divide the miles driven by the gallons used. This calculator does the division for you and also returns the same economy in L/100 km (the European standard) and km/l (common in Asia), so you can compare across regions without re-doing the math.

How to measure your actual MPG

  1. 1

    Fill the tank and note the odometer

    Fill to the first automatic shutoff, not topped off. Record either the total odometer or zero the trip meter.

  2. 2

    Drive until roughly empty

    The longer the interval, the more accurate the number. A single short commute is too noisy.

  3. 3

    Fill up again and note gallons

    Fill to the same first-shutoff level. Record gallons dispensed from the pump receipt.

  4. 4

    Divide miles by gallons

    MPG = miles driven ÷ gallons used. The calculator also returns L/100 km and km/l equivalents.

MPG, L/100 km and km/l, how they relate

The three common fuel economy units measure the same thing in inverted forms, which is why L/100 km and MPG feel reversed. More MPG is better. Fewer L/100 km is better.

Conversion quick reference

MPG (US) L/100 km km/l What it feels like
12 19.6 5.1 Large truck, towing, RV
18 13.1 7.7 Full-size SUV, older pickup
22 10.7 9.4 Mid-size sedan, mid-size SUV
28 8.4 11.9 Compact sedan, efficient SUV
35 6.7 14.9 Economy hatchback
45 5.2 19.1 Hybrid
55 4.3 23.4 Prius-class hybrid in city driving
100+ MPGe 2.4 or less 42+ Modern EV (energy-equivalent)

US gallons vs imperial gallons

A US gallon is 3.785 liters. An imperial gallon (UK, some Caribbean) is 4.546 liters, about 20% larger. A car rated 40 MPG (US) is rated ~48 MPG (imperial). Always check which gallon a published figure uses before comparing.

Why your actual MPG differs from the window sticker

  • Short trips. Cold engines run rich for the first mile or two, a commute of 2 miles kills economy.
  • Highway speeds. Aerodynamic drag grows with the square of speed. 80 mph is 25–30% worse than 65 mph on most cars.
  • Cargo and elevation changes. Every 100 lb of cargo costs about 1% in MPG on average. Climbing a pass costs a lot, coasting down gets part of it back.
  • Tire pressure. Under-inflated tires cost 0.2% MPG for every 1 psi below spec.
  • Fuel grade and ethanol content. E85 (85% ethanol) yields roughly 25% fewer MPG than pure gasoline.

When to use each unit

  • MPG: US and UK consumer facing.
  • L/100 km: EU, Canada (official), Australia. Universal in manufacturer specs.
  • km/l: Japan, India and most of Latin America.

Long-term logging

A single tank is noisy. For a reliable personal average, log 5–10 consecutive tanks and take the mean. That smooths out one cold week, one mountain trip, and one round of aggressive driving.

Frequently Asked Questions

Usually optimistic by 1–3 MPG on most vehicles because it is calibrated to round favorably. For an objective number, calculate from pump-measured gallons and odometer miles. That is also the figure insurance and fuel-reimbursement programs treat as authoritative.

Yes. MPG applies the same way regardless of fuel type, miles driven divided by volume consumed. A diesel pickup might record 19 MPG while a comparable gas pickup records 14 MPG, and the diesel figure is directly comparable.

For straight hybrids, MPG still applies. For plug-in hybrids, the published figure is often MPGe when running on battery, miles per gallon of energy equivalent. This calculator works for whichever fuel measurement you actually log: if you record gasoline consumed, you get gasoline MPG.

Divide 235.215 by the MPG (US) value. 30 MPG = 235.215 / 30 = 7.84 L/100 km. The calculator shows both so you do not need the formula.

No. A single-trip calculation is done locally and nothing is saved. For ongoing tracking, log tanks in a spreadsheet and run this for each one.

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