Gas Mileage Calculator
The tank's honest measure — two fill-ups and the gallons between them into real MPG, the metric twin, and cents per mile.
Gas Mileage Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- US gallons and miles — the converters own metric translation
- Odometer 0 to 10,000,000; gallons 0.01 to 100; price 0 to 10
In short: 52,860 minus 52,300 is 560 miles on 11.2 gallons — 50.0 MPG by the full-to-full method, the only measure that reflects your actual driving. The metric twin reads 4.7 L/100km, and at $3.50 a gallon the car costs 7¢ a mile.
Formula
MPG = (odometer 2 − odometer 1) ÷ gallons to refill; L/100km = 235.215 ÷ MPG
The full-to-full method is the gold standard because it needs no assumptions: fill the tank completely, drive normally, fill completely again. The gallons on the second receipt are exactly what the miles between the two odometer readings burned. Fill to the same click-off both times and never top off — the method's only rule is that both tanks start identical. One tank varies by 3 to 5 MPG with weather and route; three to five tanks averaged is the number to trust.
Worked Example
- Fill the tank completely and note the odometer.
- Drive normally; at the next fill-up note the odometer and the gallons the pump took.
- Read your real MPG, the metric twin, and cents per mile.
Defaults: 52,300 to 52,860 on 11.2 gallons — 560 miles, 50.0 MPG, 4.7 L/100km, 7¢ a mile at $3.50.
Strengths & Limits Of This Model
Where this engine is strong
- Full-to-full method, the honest standard
- The L/100km twin with its backwards scale named
- Cents per mile for the comparison
Where it stops
- Two fill-ups of patience required — no instant estimate
Practical Use Cases
Tank tracking
the number the dash rounds
Maintenance watch
a sudden drop names a cause
Car shopping
the real figure against the sticker
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Gas Mileage Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why fill the tank completely twice?
Because the pump only tells you what went IN, not what the engine BURNED. With both tanks filled to the same click-off, the refill equals the burn since the last fill — no guesswork about where the fuel level sat.
Why does my number miss the EPA sticker?
The sticker is a lab cycle; the road adds short trips, cold starts, idling, roof racks and seasons. Real-world MPG running below the estimate is normal, and a few MPG of gap is not a fault.
Why not trust the dash average?
The in-dash number is computed from the same arithmetic but tuned optimistic and reset-happy. It is fine for watching a trend; the two-receipt measure is the one that survives an argument.
What does a sudden MPG drop mean?
Usually maintenance: low tire pressure, a lazy oxygen sensor, a dragging brake, a dirty air filter. Tracking three to five tanks makes the step visible; the pages' arithmetic is how the step gets measured.
What is L/100km and why backwards?
The metric convention burns litres per hundred kilometres, so SMALLER is better — the reverse of MPG, where bigger wins. The card prints the conversion (235.215 over your MPG) so the two scales can be read side by side.
Can I use trip-odometer miles instead?
Yes — reset the trip meter at the first full fill-up and enter its reading as the distance. The main odometer is the safer habit only because trip meters get reset by accident.
Should I top off after the click?
No — topping off corrupts both the measure and the tank's vapour system. The method's rule is the same click-off both times; the pump knows when to stop.
Why does the price field default to a number?
The cents-per-mile card is the line that compares cars and commutes, and it needs the pump price. Set it to zero and the card reads an honest 0¢ — the MPG cards never depend on it.