Gas Mileage Calculator

The EPA's window-sticker MPG comes from standardized lab tests run on a dynamometer. The numbers are reasonable averages but they assume specific driving patterns: a particular acceleration profile, a fixed speed range, and lab-controlled conditions. Real-world MPG often runs 10–25% below the sticker, especially in stop-and-go urban driving, cold weather, or mountainous terrain.

The mismatch is widest for hybrids and EVs, whose efficiency depends heavily on regenerative braking opportunities (better in cities) and battery temperature (worse in cold). A Toyota Prius rated 56 MPG combined often returns 50–55 MPG in moderate climates and 40–45 MPG in cold-weather highway driving. Tracking your own car's actual MPG month over month tells you what to expect; the sticker is a rough starting point.

Speed: most modern gasoline cars peak in efficiency at 45–55 mph. Above 60 mph, MPG drops noticeably from aerodynamic drag (which scales with the square of velocity). Going 75 instead of 65 mph typically reduces fuel economy by 10–15%, with no time savings on most real-world commutes.

Aggressive driving (rapid acceleration, hard braking) reduces MPG by 15–30% in city driving and 10–20% on highways. Smooth acceleration and anticipating stops matter more than most drivers realize — fleet operators routinely improve their fuel economy 10%+ through driver-coaching programs alone.

Tire pressure: under-inflated tires increase rolling resistance and reduce MPG. The effect is modest individually (~0.2% per 1 PSI under spec) but cumulative — chronically running 5–8 PSI low can cost several MPG, plus accelerate tire wear.

Weight and cargo: each 100 lbs of unnecessary cargo costs 1–2% in fuel economy. A roof rack adds aerodynamic drag worth 5–8% MPG even when empty. Removing both when not needed is one of the simplest efficiency wins.

The basic trip-cost formula: cost = (distance / MPG) × fuel price. A 400-mile trip in a 25-MPG car at $3.50/gallon: 16 gallons × $3.50 = $56. The same trip in a 40-MPG car: 10 gallons × $3.50 = $35. Over 15,000 annual miles, the MPG difference is $787/year at that fuel price — meaningful when comparing vehicles to buy.

Hybrid and EV break-even: the higher upfront cost vs. typical sedan can be recouped through fuel savings, but the break-even depends on annual miles, fuel price, electricity rates, and how long you keep the vehicle. A $4,000 hybrid premium with $700/year fuel savings recoups in 5.7 years — fine if you keep the car 8–10 years, marginal if you sell at 5.

An unexplained drop in MPG often signals a maintenance issue: a failing oxygen sensor, a stuck thermostat, dragging brakes, an engine air filter past its useful life, or a misfiring cylinder. Tracking MPG fill-up over fill-up reveals these problems early, sometimes before they trigger a check-engine light.

Routine maintenance — clean air filter, properly inflated tires, on-spec oil viscosity, working oxygen sensors — generally produces 5–10% better MPG than a neglected vehicle. The savings often pay for the maintenance.