Calculate your vehicle's real-world fuel economy or estimate how much gas your next road trip will cost. Enter your miles driven and gallons used to get your MPG, or switch to trip-cost mode to plan fuel expenses — all updated in real time.
Pro tip: Your car's real-world MPG is almost always lower than the EPA rating. City driving, AC use, aggressive acceleration, and underinflated tires each reduce fuel economy by 5–15%. Track your actual MPG over 3–4 fill-ups for an accurate number.
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.