Gas Mileage Calculator
Calculate your vehicle's real-world fuel economy in MPG or L/100km. Enter distance and fuel used to track efficiency, cost per mile, and full-tank range.
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Enter your values above to see the results.
Tips & Notes
- ✓For the most accurate MPG reading, always fill to the pump click-off point at both the start and end of your measurement period — partial fills introduce error.
- ✓Highway driving typically yields 20-30% higher MPG than city driving. Calculate both separately to understand your real-world efficiency in each condition.
- ✓Track MPG over at least 3-4 consecutive fill-ups to get a reliable average. A single fill-up result can be skewed by a particularly efficient or inefficient driving period.
- ✓A sudden MPG drop of 10% or more between fill-ups often signals a maintenance issue: low tire pressure, a dirty air filter, worn spark plugs, or a failing oxygen sensor.
- ✓L/100km is the standard metric — lower values mean better efficiency. A 30 MPG car uses 7.84 L/100km; a 40 MPG hybrid uses 5.88 L/100km.
Common Mistakes
- ✗Not filling to the same level at both readings — starting with a half-full tank and topping off at the end (or vice versa) makes MPG results unreliable by 5-20%.
- ✗Mixing miles and kilometers — if odometer readings are in km but you enter them as miles, your calculated MPG will be wrong by a factor of 1.609.
- ✗Entering the dollar amount paid instead of gallons used — the fuel quantity field requires gallons, not the total cost at the pump.
- ✗Using a single fill-up as a permanent baseline — one result reflects a specific driving period and conditions, not your long-term average efficiency.
- ✗Forgetting that accessories (AC, roof rack, heavy loads) measurably reduce MPG — factor these in when comparing fill-up results under different conditions.
Gas Mileage Calculator Overview
Knowing your vehicle's real-world MPG — not the EPA estimate, but your actual observed efficiency under your specific driving conditions — is the foundation of accurate fuel budgeting, vehicle comparison, and maintenance planning. A car achieving 5 MPG less than its EPA rating is spending $300-600 more per year on fuel than its owner expects.
MPG calculation formula:
MPG = Miles Driven / Gallons Used | Cost per Mile = Fuel Price / MPG
EX: Drove 312 miles, used 10.4 gallons at $3.60/gal → MPG = 312 / 10.4 = 30.0 MPG → L/100km = 235.21 / 30 = 7.84 → Cost per mile = $3.60 / 30 = $0.12/mileMPG to L/100km conversion — full reference table:
| MPG | L/100km | Cost/Mile at $3.50 | Typical Vehicle |
|---|---|---|---|
| 15 MPG | 15.68 L/100km | $0.233 | Large pickup truck, heavy SUV |
| 20 MPG | 11.76 L/100km | $0.175 | Midsize SUV (city heavy) |
| 25 MPG | 9.41 L/100km | $0.140 | Midsize sedan, average car |
| 30 MPG | 7.84 L/100km | $0.117 | Compact car, efficient sedan |
| 35 MPG | 6.72 L/100km | $0.100 | Small car, mild hybrid |
| 45 MPG | 5.23 L/100km | $0.078 | Toyota Prius, hybrid sedan |
| 55 MPG | 4.28 L/100km | $0.064 | Plug-in hybrid on gas |
| Speed (mph) | Efficiency vs. 55 mph optimum | Effect on a 30 MPG car |
|---|---|---|
| 55 mph | 100% (optimal) | 30.0 MPG |
| 60 mph | 97% (−3%) | 29.1 MPG |
| 65 mph | 92% (−8%) | 27.6 MPG |
| 70 mph | 83% (−17%) | 24.9 MPG |
| 75 mph | 77% (−23%) | 23.1 MPG |
| 80 mph | 72% (−28%) | 21.6 MPG |
Frequently Asked Questions
Use the fill-to-fill method: fill your tank completely (wait for the pump to click off), note the odometer. Drive normally. At the next fill-up, fill completely again, note the new odometer reading and the exact gallons pumped. Divide the miles driven by gallons pumped. Always use the same fill level at both points — partial fills introduce error. Repeat over 3-4 fill-ups to get a reliable average rather than relying on one data point.
EPA fuel economy tests are conducted in controlled laboratory conditions on a dynamometer, not on real roads. They do not account for traffic, hills, air conditioning, cold starts, cargo weight, or individual driving habits. Most drivers see real-world MPG 10-20% below the EPA combined estimate. Aggressive driving, frequent short trips, extreme temperatures, and heavy AC use push real-world MPG even further below the EPA figure.
Speed is the largest single variable — aerodynamic drag increases with the square of speed, so driving 75 mph instead of 65 mph increases fuel consumption by roughly 17%. Air conditioning reduces MPG by 5-25% depending on outside temperature. Under-inflated tires by 10 PSI reduce MPG by 3-4%. Cold weather hurts efficiency because cold engines run rich fuel mixtures until reaching operating temperature. Short trips are especially inefficient for this reason — a 3-mile cold-start trip may achieve only 50-60% of your highway MPG.
Divide 235.21 by MPG to get L/100km. Example: 30 MPG = 235.21 ÷ 30 = 7.84 L/100km. To convert L/100km back to MPG, divide 235.21 by the L/100km figure. Example: 8.0 L/100km = 235.21 ÷ 8.0 = 29.4 MPG. The relationship is inverse — improving from 20 to 22 MPG saves more fuel per mile than improving from 40 to 42 MPG, even though both are 2 MPG gains.
Maintain steady speeds using cruise control on highways (improves MPG 7-14%). Accelerate gradually from stops instead of aggressively (10-30% city improvement). Keep tires at the recommended PSI — check monthly, as tires lose about 1 PSI per month. Replace the air filter every 15,000-30,000 miles (improvement varies). Remove unused roof racks and cargo that adds drag and weight. Service spark plugs on schedule. Avoid unnecessary idling — an idling engine gets 0 MPG.
MPG directly determines your fuel budget. At 15,000 miles/year and $3.50/gallon: a 25 MPG car costs $2,100/year in fuel; a 35 MPG car costs $1,500/year — a $600 annual difference. Over 5 years, that is $3,000 in fuel savings from choosing a more efficient vehicle. MPG also affects insurance (more driving = higher risk) and maintenance intervals. Tracking your personal MPG over months reveals both efficiency trends and early warning signs of mechanical issues that affect fuel economy.