How to Calculate Gas Mileage
Calculating your gas mileage is simple: divide the miles you drove by the gallons of fuel you used. This gas mileage calculator automates the process and also gives you L/100km, cost per mile, and total fuel cost. Whether you call it a gas mileage calculator, MPG calculator, or fuel economy calculator, the formula is the same:
For metric users, the equivalent is L/100km = (Liters × 100) ÷ Kilometers. The conversion shortcut between MPG and L/100km is 235.215 ÷ value—so 27.5 MPG equals about 8.55 L/100km.
The Fill-to-Fill Method (Most Accurate)
- Fill your tank completely and note the odometer reading (or reset your trip meter).
- Drive normally until your next fill-up.
- Fill up again and record the gallons pumped and new odometer reading.
- Enter the numbers above—our gas mileage calculator does the rest instantly.
See a worked example
Odometer start: 12,340.5 → end: 12,690.2 gives 349.7 miles. Fuel added: 12.7 gal at $3.79/gal.
- MPG = 349.7 ÷ 12.7 = 27.53 MPG
- L/100km = 235.215 ÷ 27.53 = 8.55 L/100km
- Cost per mile = $3.79 ÷ 27.53 = $0.138/mi
- Total cost = 12.7 × $3.79 = $48.13
Fuel Economy Quick Facts
Key statistics from the EPA and U.S. Department of Energy to help you understand where your vehicle stands.
Our Free Calculator Tools
Ten free fuel economy and cost calculators—pick the one that fits your need, or use them together for a complete picture of your driving costs.
Calculate MPG or L/100km from odometer readings or trip distance with fuel cost breakdown.
Use calculator →Estimate your total trip fuel cost from distance, fuel economy, and gas price per unit.
Use calculator →In-depth MPG calculator with fuel economy tips, EPA data, and cost-per-mile analysis.
Use calculator →Estimate total fuel cost for any road trip from distance, MPG, and gas price.
Use calculator →Compare annual fuel costs between gas and electric vehicles to see potential savings.
Use calculator →Calculate daily, weekly, monthly, and annual fuel costs for your regular commute.
Use calculator →Compare fuel costs between two vehicles to see potential annual, monthly, and 5-year savings.
Use calculator →Calculate fuel cost per mile and total operating cost per mile including maintenance, insurance, and depreciation.
Use calculator →Calculate how far you can drive on a full tank based on tank size, MPG, and current fuel level.
Use calculator →Calculate how much you can save by carpooling, including annual savings per person and CO₂ reduced.
Use calculator →Fuel Economy Guides & Tips
Learn how to improve your gas mileage, reduce fuel costs, and make smarter driving decisions with our in-depth guides.
How Do You Calculate Your Miles Per Gallon?
Learn the exact formula and step-by-step method to calculate MPG, with worked examples, EPA testing explained, and 12 tips to improve fuel economy.
Read guide →How to Calculate Fuel Cost for a Road Trip
Use the simple formula (Distance ÷ MPG) × Price Per Gallon to estimate your road trip fuel cost with worked examples and money-saving tips.
Read guide →What Is Good Gas Mileage for a Car?
Good gas mileage depends on vehicle type: 30+ MPG for sedans, 25+ for SUVs, and 20+ for trucks. See 2024 EPA averages and benchmarks.
Read guide →Why Is My Car Getting Bad Gas Mileage?
Discover 12 common causes of poor fuel economy—from underinflated tires to faulty sensors—plus a troubleshooting checklist.
Read guide →How to Save Money on Gas
Cut your annual fuel bill with efficient driving habits, vehicle maintenance, smart fueling strategies, and road trip planning tips.
Read guide →MPG to L/100km Conversion Guide
Convert between MPG, L/100km, and km/L with the simple 235 shortcut. Includes tables and tips for international travelers.
Read guide →Best Gas Mileage Cars 2026
Discover the most fuel-efficient cars for 2026, from hybrids getting 55+ MPG to the best sedans and SUVs with EPA ratings.
Read guide →How Much Does It Cost to Drive 1000 Miles?
Find out the cost to drive 1000 miles based on your MPG and gas prices, with cost tables and worked examples.
Read guide →Does Air Conditioning Use More Gas?
AC reduces fuel economy by 5–25%. Learn when to use AC vs windows and how to minimize the MPG penalty.
Read guide →Highway vs City MPG Explained
Why highway MPG is 20–30% higher than city MPG, how EPA tests both, and which number matters for you.
Read guide →Is Premium Gas Worth It?
Premium gas only benefits cars that require it. Learn when it matters, when it doesn't, and how much you could save.
Read guide →Browse by topic: MPG Basics | Trip Cost | EV Efficiency | Maintenance
Frequently Asked Questions
Common questions about calculating gas mileage, improving fuel economy, and using our tools.
How do I calculate my car’s gas mileage?
Fill your tank, note the odometer, drive normally, then fill up again. Divide the miles driven by the gallons pumped. For example, 350 miles ÷ 13 gallons = 26.9 MPG. Our gas mileage calculator above does this instantly.
Is my car’s dashboard MPG display accurate?
Dashboard MPG displays tend to be optimistic by 5–15%. The manual fill-up method is more reliable for tracking actual fuel economy over time. Learn more in our guide on how to calculate miles per gallon.
What is good gas mileage for a car?
The US fleet average is 27.2 MPG (2024 EPA). Sedans typically get 30–35 MPG, SUVs 25–30 MPG, and trucks around 20 MPG. Anything above 30 MPG is generally considered fuel-efficient. Read our full breakdown: What is good gas mileage?
Why did my gas mileage suddenly drop?
A sudden MPG drop often signals underinflated tires, a dirty air filter, a faulty oxygen sensor, or a change in driving conditions (cold weather, more city driving). A failing O2 sensor alone can reduce MPG by up to 40%. See all 12 causes in our guide: Why is my car getting bad gas mileage?
How do I calculate gas cost for a road trip?
Divide your trip distance by your MPG to get gallons needed, then multiply by the gas price. For example: 500 miles ÷ 25 MPG = 20 gallons × $3.50 = $70. Use our trip gas cost calculator for instant estimates with round-trip support.
Does AC or speed affect MPG?
Yes. AC can reduce fuel economy by 5–25%. Driving above 50 MPH reduces efficiency significantly—every 5 MPH over 50 is like paying an extra $0.25/gallon. Aggressive acceleration is the single biggest MPG killer. Learn more tips in our guide to saving money on gas.
How do I convert MPG to L/100km?
Divide 235.215 by your MPG number. For example: 30 MPG = 235.215 ÷ 30 = 7.84 L/100km. Remember, lower L/100km means better fuel economy. See our full MPG to L/100km conversion guide.
Is an electric car cheaper to run than a gas car?
In most cases, yes. A typical EV costs about $500–$600/year in electricity versus $1,500–$1,800/year in gas for a comparable vehicle. Use our gas vs electric car cost calculator to compare costs based on your specific driving habits and local energy prices.