This MPG calculator computes your vehicle’s real fuel economy from a fill-up — miles driven and gallons used — with cost per mile and the metric L/100km conversion your dashboard may not show.
The Fill-Up Method (the Only Honest MPG)
Dashboard readouts estimate; the pump measures. The correct protocol: fill the tank completely and reset the trip odometer; drive normally; at the next fill-up, fill completely again and note both the trip miles and the gallons the pump dispensed. MPG = miles ÷ gallons — you have then measured everything the engine burned over that distance, no modeling involved. Average three tanks before judging: single tanks wobble with pumps that click off early, terrain and weather. This calculator is part of our Everyday Life Calculators collection.
Worked Example (Verified)
- 300 miles on 10.0 gallons → 30.0 MPG
- Metric equivalent: 235.215 ÷ 30 = 7.84 L/100km (note the inversion: for L/100km, lower is better)
- At $3.50/gallon: fuel cost = 3.50 ÷ 30 = $0.117 per mile — $11.67 per 100 miles, the number that makes road-trip budgets real
Why Your MPG Differs From the Sticker
- EPA ratings are lab cycles: real traffic, cold starts, A/C, cargo and 75-mph highways all subtract. Falling 10–20% short of the combined rating is normal, not a defect.
- Speed: aerodynamic drag grows with the square of speed — most cars peak in efficiency at 45–55 mph and lose roughly 10–15% by 75 mph.
- UK gallons differ! An Imperial gallon is 1.201 US gallons, so UK-quoted MPG reads ~20% higher for identical economy (30 US MPG = 36 UK MPG). This calculator uses US gallons.
- A sudden persistent drop (15%+) is diagnostic: underinflated tires, a clogged air filter, dragging brakes or a failing sensor — MPG is your cheapest engine-health monitor.
Related Tools
- Everyday Life Calculators — the full collection
- Discount Calculator — for the other side of the pump price
Frequently Asked Questions
How do I calculate MPG?
Miles driven ÷ gallons used between two full fill-ups. 300 miles ÷ 10 gallons = 30 MPG. Reset the trip odometer at each fill to make it effortless.
How do I convert MPG to L/100km?
Divide 235.215 by the US MPG figure: 30 MPG = 7.84 L/100km. The measures are reciprocal in spirit — high MPG is efficient, low L/100km is efficient.
What is a good MPG?
Context rules: ~25 MPG is typical for a gasoline SUV, 30–35 for a sedan, 40–55 for hybrids, and 15–20 for trucks. Compare against your vehicle’s EPA combined rating rather than universal benchmarks.
Why is my winter MPG worse?
Cold engines run rich longer, winter fuel blends carry slightly less energy, tires lose pressure, and short trips never fully warm up — a combined 10–15% seasonal dip is unremarkable.
Does this calculator work for electric cars?
The gallons side does not apply; EVs use MPGe or kWh/100mi. (Cost-per-mile comparisons still work if you divide your electricity price per kWh by miles per kWh.)
Is this MPG calculator free?
Yes — free, instant, no sign-up.