Real-World MPG Calculator (Fill-Up Method)

It estimates fuel from injector pulse-widths and assumed fuel density, both of which drift with temperature, fuel blend and injector age — and manufacturers calibrate toward the flattering side of the tolerance. Studies and owner-fleet data (Consumer Reports, fuelly-style logs) consistently find 3–6% optimism, occasionally 10%. The pump-and-odometer method has one assumption: that 'full' means the same thing twice. That's why EPA's own 'Your MPG' program uses it.

At least three, averaged — single-tank readings swing ±2–3 MPG from fill variation alone (nozzle click point, pump angle, temperature expansion in the tank). Track total miles ÷ total gallons across a month and the noise collapses. Watch the rolling average, not the individual tank: a genuine 2-MPG decline sustained over five tanks is a real signal (tire pressure, dragging brake, O2 sensor, winter blend) worth investigating.

A stack of small physics taxes: winter-blend gasoline carries ~1.5–3% less energy per gallon, cold engines run rich for the first miles, idle warm-up burns fuel at 0 MPG, cold lubricants drag, dense cold air raises aero drag, and winter tires add rolling resistance. Net 10–20% worse in cold climates — fully normal. Short trips amplify it because the engine never escapes warm-up enrichment; the same car on a winter highway run loses far less.

Tire pressure first (each 5 psi under costs ~1–2%; check monthly, cold), then speed habits — aerodynamic drag grows with speed squared, so 75→65 mph typically saves 10–15% on highway legs. Roof boxes are economy assassins (10–25% at speed). Premium fuel in an engine that doesn't require it buys nothing measurable. Re-measure with three tanks after each change and let this calculator referee the claims.