HowMuchIt / Gas for a trip
How much gas will my trip use?
Divide the miles by your car's miles per gallon. The calculator does it, doubles it for a round trip, and multiplies by the price you see at the pump.
Work out the gas for your trip
Gallons = miles / MPG. At highway speeds above about 50 to 55 mph, real MPG falls below the rating: type your own measured MPG if you have it.
Your MPG, not the sticker's
The EPA estimate is a fair comparison between cars, but your own MPG can differ with your driving, the weather and the load. To measure it, fill the tank, drive, fill it again, and divide the miles driven by the gallons you added. Fuel economy also drops quickly above about 50 to 55 miles per hour, so a fast highway trip uses more gas than the rating suggests: use the low end of your own range for long highway days.
What eats into a trip's MPG
Aggressive driving, with speeding, fast starts and hard braking, can lower gas mileage by about 15 to 30 percent at highway speeds and 10 to 40 percent in stop-and-go traffic. Every extra 100 pounds in the car costs about 1 percent. A large rooftop cargo box can cut mileage by roughly 10 to 25 percent at 65 to 75 mph, so take it off when you do not need it. Tires low by 1 psi cost about 0.2 percent each, and idling burns about a quarter to half a gallon an hour.
Questions people ask
How many gallons for 300 miles?
At 25 MPG, 12 gallons; at 30 MPG, 10.
Why is my MPG lower than the EPA's?
Speed, driving style and load change it; measure yours at the pump.
How much gas does idling use?
About a quarter to half a gallon an hour.
Good to know
- MPG is miles driven per gallon of fuel, so the gallons a trip needs = miles / MPG.1,2
- Fuel economy drops quickly once you drive faster than about 50 to 55 mph, so a fast highway trip uses more gas than the rated MPG suggests.2,3,4
- Your own MPG can differ from the EPA estimate; to measure it, fill the tank, drive, refill, and divide the miles by the gallons added.2,5
- Aggressive driving (speeding, fast starts, hard braking) can lower gas mileage by about 15 to 30 percent at highway speeds and 10 to 40 percent in stop-and-go traffic.6,7,8
- Each extra 100 pounds in the car lowers MPG by about 1 percent, so leave heavy items you don't need at home.6,7,9
- A large rooftop cargo box can cut gas mileage by roughly 10 to 25 percent at 65 to 75 mph.6,7,10
- Take a roof carrier or cargo box off when you are not using it.6,9
- Under-inflated tires lower gas mileage, by about 0.2 percent for every 1 psi the tires are low.7,9,11
- Idling burns about a quarter to a half gallon of fuel per hour, depending on the engine and air conditioning use.6,12
How we checked this page
Checked on October 27, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- Fuel economy in automobiles (unit definition only), Wikipedia
- Gas Mileage Calculator, Calculator.net
- Driving More Efficiently, fueleconomy.gov (DOE/EPA)
- Fuel-efficient driving techniques, Natural Resources Canada (Government of Canada)
- Frequently Asked Questions, fueleconomy.gov (DOE/EPA)
- Driving More Efficiently, fueleconomy.gov (DOE/EPA)
- How to Squeeze the Most Out of Every Drop of Gas, The Motley Fool (The Ascent)
- Aggressive driving costs consumers money at the gas pump, ConsumerAffairs
- Fuel Saving Tips, AAA
- How Rooftop Carriers Affect Fuel Economy for Cars and SUVs, Consumer Reports
- Keeping Your Vehicle in Shape, fueleconomy.gov (DOE/EPA)
- Stop Idling: How Turning Off Engine Saves Fuel, Kelley Blue Book (Cox Automotive)