Gas Mileage CalculatorMPG Calculator & Real-World Fuel Economy Check
Gas mileage (MPG) = miles driven ÷ gallons used.
Find your car’s true miles per gallon from a fill-up or odometer readings, then see your cost per mile, L/100 km, and how far you are from the EPA rating — plus what closing that gap would save you each year.
Your numbers
From the window sticker or FuelEconomy.gov. Leave 0 to skip.
That is 6.7% below the 30 MPG EPA combined rating.
6.7% below EPAYour MPG vs EPA rating
How do you calculate gas mileage?
Gas mileage (MPG) = miles driven ÷ gallons used. Fill the tank, reset your trip meter, drive normally, then refill and divide the miles by the gallons it took. For example, 350 miles on 12.5 gallons is 28.0 MPG. Divide the gas price by MPG for cost per mile — $3.20 ÷ 28 = about 11.4 cents a mile.
Part of our Fuel, Mileage & Trip Cost calculators — for the core estimate, start with the Fuel Cost Calculator.
How to calculate MPG with the fill-up method
Fill up completely
Fill the tank until the pump clicks off. Don’t top off — consistency matters more than squeezing in extra.
Reset the trip meter
Zero your trip odometer, or write down the main odometer reading.
Drive normally
Use at least half a tank in your usual mix of city and highway driving.
Refill and divide
Fill up at the same pump if you can. Divide miles driven by the gallons it took to refill.
Your dashboard’s MPG readout is an estimate and is often a little optimistic. The fill-up method measures what actually went into the tank, which is why it’s the most reliable way to check real-world gas mileage. Average two or three tanks for a figure you can trust.
Gas mileage formula and conversions
Miles driven = current odometer − odometer at last fill-up
Cost per mile = price per gallon ÷ MPG
Yearly fuel cost = (miles per year ÷ MPG) × price per gallon
L/100 km = 235.215 ÷ US MPG · km per litre = US MPG × 0.425144
The 235.215 constant converts between US gallons and litres and between miles and kilometres in one step. If you’re working with UK (imperial) gallons, use 282.481 instead.
Once you know your real MPG, plug it into the Fuel Cost Calculator to price any trip, or convert it to litres per 100 km with the Fuel Consumption Calculator.
Example: checking a 30 MPG car
The fill-up
Odometer went from 24,150 to 24,500 = 350 miles.
Refill took 12.5 gallons at $3.20.
MPG = 350 ÷ 12.5 = 28.0 MPG
What it means
Cost per mile = $3.20 ÷ 28 = $0.114.
That’s 6.7% below the 30 MPG EPA rating.
At 12,000 miles a year: $1,371.43 in fuel, versus $1,280.00 at the EPA rating.
These are the calculator’s default numbers, so you can check every result above against this example.
MPG to L/100 km conversion chart
| US MPG | L/100 km | km per litre | UK MPG | Fuel cost per 100 mi at $3.20 |
|---|---|---|---|---|
| 15 | 15.68 | 6.38 | 18.0 | $21.33 |
| 20 | 11.76 | 8.50 | 24.0 | $16.00 |
| 25 | 9.41 | 10.63 | 30.0 | $12.80 |
| 30 | 7.84 | 12.75 | 36.0 | $10.67 |
| 35 | 6.72 | 14.88 | 42.0 | $9.14 |
| 40 | 5.88 | 17.01 | 48.0 | $8.00 |
| 45 | 5.23 | 19.13 | 54.0 | $7.11 |
| 50 | 4.70 | 21.26 | 60.0 | $6.40 |
Why your real MPG is lower than the EPA rating
EPA ratings come from standardized lab tests, so they’re best for comparing vehicles rather than predicting your exact result. FuelEconomy.gov notes that real-world mileage varies with how, where and in what conditions you drive. The usual culprits:
- Speed: fuel economy typically drops quickly above about 50 mph as aerodynamic drag rises.
- Aggressive driving: hard acceleration and braking waste fuel, especially in stop-and-go traffic.
- Short trips and cold weather: engines run less efficiently until they warm up.
- Under-inflated tires: more rolling resistance means more fuel per mile.
- Extra weight and roof racks: cargo boxes and bike racks add drag at highway speeds.
- Idling and A/C use: both burn fuel without moving you forward.
Common MPG calculation mistakes
- Using one tank only. Pump shut-off points vary; average two or three fill-ups.
- Trusting the dash display. Trip computers estimate; the fill-up method measures.
- Mixing partial fills. If you didn’t fill to the top, carry the miles over to the next full fill.
- Comparing city-only driving to the combined rating. Compare city driving to the EPA city figure instead.
Thinking about switching to electric? The EV vs Gas Calculator compares your measured cost per mile with an EV’s, including the break-even year.
Related calculators
Gas Mileage Calculator FAQs
Fill your tank, reset the trip meter, drive normally, then refill. Divide the miles driven by the number of gallons it took to refill. For example, 350 miles ÷ 12.5 gallons = 28 MPG.
It depends on the vehicle class. Compact cars and hybrids commonly rate in the 30s to 50s MPG combined, while pickups and large SUVs are often in the high teens to mid-20s. The most useful comparison is your measured MPG against your own car’s EPA rating.
Divide the price per gallon by your MPG. At $3.20 a gallon and 28 MPG, fuel costs about $0.114 per mile, or $11.43 every 100 miles.
EPA ratings come from standardized lab tests. Real driving adds higher speeds, hard acceleration, cold starts, short trips, A/C use, under-inflated tires and extra weight, all of which lower fuel economy.
Trip computers estimate fuel use from injector data and are often a few percent optimistic. The fill-up method, which measures fuel actually pumped, is more accurate.
Divide 235.215 by the MPG figure. For example, 28 MPG is 235.215 ÷ 28 = 8.40 L/100 km. For UK gallons, divide 282.481 by the UK MPG.
At least two or three. Pumps shut off at slightly different points, so a single tank can be off by a few percent either way.
Usually, yes — steady speeds avoid the energy lost to stopping and starting. But economy falls off at high speeds, so 75 mph typically uses noticeably more fuel per mile than 60 mph.
Yes. Subtract the odometer reading at your last full fill-up from today’s reading to get miles driven, then divide by the gallons it took to fill up again.
Yes. Taller tires make the odometer under-record miles, so your calculated MPG will be lower than reality unless you correct the distance for the new tire size.
About the developer
Manzoor Ahmad
Web Designer & Calculator Developer · 13+ years of web development and SEO experienceI’m a professional pharmacist turned tech geek, and I designed and coded this gas mileage calculator myself. I’m not an automotive engineer, so the tool uses the fill-up method and conversion constants published by FuelEconomy.gov, and the worked example on this page uses the exact default numbers so you can check every result by hand.
Found a number that looks wrong, or want a feature added? Tell me through my author page — corrections are fixed and re-dated.
Sources
- U.S. DOE & EPA — FuelEconomy.gov
- U.S. DOE & EPA — Driving more efficiently
- U.S. EPA — Fuel economy and environment label explained
- U.S. Energy Information Administration — Gasoline and diesel fuel update
Disclaimer: Results reflect the numbers you enter. Real fuel economy varies with driving style, weather, terrain, vehicle condition and fuel blend. EPA ratings are for comparison and your mileage will vary.
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