About the MPG Calculator
The MPG Calculator computes real-world fuel economy from miles ÷ gallons (or kilometers ÷ liters), supports both U.S. (MPG) and metric (L/100km) outputs, and adds three companion modes: trip cost (distance ÷ MPG × price per gallon), annual fuel budget (yearly miles projected forward), and CO&sub2; emissions (gallons × EPA standard ~8.89 kg CO&sub2; per gallon of gasoline).
It is built for owners auditing whether their car actually delivers the EPA-sticker MPG, road-trippers budgeting fuel before a long drive, gig drivers calculating cost per mile against their pay, fleet managers stress-testing a vehicle swap, climate-curious commuters wanting the emissions number, and anyone whose dashboard MPG display they suspect is optimistic (it usually is).
All calculations run locally in JavaScript. Mileage, fuel-used, gas-price, and trip inputs never leave your device. The page makes no network call after first load. No GPS or vehicle telemetry is read; the calculator only sees what you type.
EPA sticker MPG is measured on a dyno under standardized conditions and almost always overstates real-world economy by 10–20% for typical drivers. Aggressive acceleration, hard braking, cold weather (winter gasoline plus engine warm-up), high-altitude crossings, rooftop cargo, and underinflated tires each cost 5–20% individually. Track 3–4 full tanks at the same gas station and average the result for a number you can plan against; one tank is too noisy.
Log multiple fill-ups to track your running average MPG over time.
Compare two vehicles side-by-side to see the annual cost difference.
See how driving behavior changes affect your MPG and annual savings.
How to Calculate Your Car's MPG
Calculating your car's actual fuel economy is straightforward. Fill up your tank completely, reset your trip odometer (or note the mileage), then drive normally until you need gas again. At the pump, fill up completely once more and note how many gallons it took. Divide the miles you drove by the gallons you used — MPG = Miles Driven ÷ Gallons Used. For the most accurate reading, repeat this process over three to four fill-ups and average the results. A single tank can be misleading because of temperature swings, route changes, or differences in how full the pump leaves the tank. This calculator does the math for you instantly and tracks your running average if you subscribe.
MPG vs L/100km: Converting Fuel Economy Units
The United States measures fuel economy in miles per gallon (MPG), while most of the world uses litres per 100 kilometres (L/100km). The two scales run in opposite directions — a higher MPG means better efficiency, while a lower L/100km is better. To convert, divide 235.215 by the value in the other system: L/100km = 235.215 ÷ MPG, and MPG = 235.215 ÷ L/100km. A car rated at 30 MPG uses about 7.8 L/100km. This calculator handles conversions automatically when you toggle between miles/gallons and km/litres, so you can compare vehicles rated in different systems without manual arithmetic.
What Affects Your Gas Mileage?
Dozens of factors influence real-world fuel economy. Driving speed is the biggest lever: aerodynamic drag increases with the square of velocity, so cruising at 75 mph instead of 65 can drop your MPG by 10–15%. Aggressive acceleration and hard braking waste the kinetic energy your engine worked to create, reducing city MPG by up to 33%. Tire pressure matters more than most drivers realise — every 1 psi below the recommended level costs roughly 0.2% in fuel economy. Running the air conditioner at full blast can reduce MPG by 3–4% on the highway and even more in stop-and-go traffic. Vehicle weight counts too: every extra 100 pounds reduces MPG by about 1%. Finally, cold weather thickens engine oil, increases tire rolling resistance, and forces longer warm-up times, which is why winter fuel economy is noticeably worse than summer.
How to Estimate Road Trip Fuel Costs
Planning a road trip? Switch this calculator to Trip Cost mode and enter the round-trip distance, your vehicle's MPG (use your tracked average if you have one), and the current price of gas. The formula is simple: Fuel Cost = (Distance ÷ MPG) × Price Per Gallon. Keep in mind that highway driving typically delivers better MPG than your combined rating, while mountain passes and headwinds will lower it. If you are comparing driving to flying, remember to include tolls, parking, and extra meals on the road. For multi-stop trips, add each leg separately since your MPG may vary significantly between flat interstate stretches and winding back roads.
Average MPG by Vehicle Type
Fuel economy varies widely by vehicle class. Compact sedans and hatchbacks typically achieve 30–40 MPG combined, making them the most economical choice for daily commuters. Midsize sedans land around 27–35 MPG, while full-size sedans drop to 23–30 MPG. Crossover SUVs range from 25–32 MPG, but full-size SUVs and trucks fall to 15–22 MPG due to their weight and aerodynamic profile. Conventional hybrids push 45–60 MPG, and plug-in hybrids can achieve the equivalent of 80–100+ MPGe on short trips using battery power alone. If your result falls well below the average for your vehicle class, that is a signal to check tire pressure, air filter condition, and driving habits — small fixes that collectively recover 5–15% of lost fuel economy.
Looking for related tools? Try our Fuel Cost Calculator for detailed fuel budgets, or explore all Everyday Calculators.
Frequently Asked Questions
How do I calculate MPG?
Divide the miles you drove by the gallons of fuel you used. If you drove 300 miles on 12 gallons, your MPG is 25. The calculator does this instantly and also supports the metric equivalent of liters per 100 kilometers.
Why is my real MPG lower than my EPA rating?
EPA ratings are measured on a dyno under controlled conditions using a standardized drive cycle. Real-world driving is almost always 10% to 20% worse due to traffic, terrain, weather, AC use, roof racks, and aggressive acceleration. Tracking your actual MPG is the only way to get a true number.
What is a good MPG for a car?
As a rough benchmark, a modern compact or hybrid sedan should hit 35 to 50 MPG combined, a mid-size sedan around 28 to 34 MPG, a small SUV 25 to 30 MPG, and a full-size truck 17 to 22 MPG. Hybrids and plug-in hybrids typically top those ranges.
Does driving style really affect MPG?
Yes, by a surprising amount. Aggressive acceleration and hard braking can reduce fuel economy by 15% to 30% in city driving. Speeding over 55 to 60 mph cuts highway MPG too, since aerodynamic drag rises with the square of speed.
Is MPG the same as L/100km?
No, they are inverse scales. Higher MPG is better; lower L/100km is better. 30 MPG equals roughly 7.84 L/100km, and 40 MPG equals about 5.88 L/100km. Because the relationship is not linear, gains in the low-MPG range save more fuel than equal gains in the high range.