Gas Savings from Better MPG Calculator: Yearly Fuel Savings, Payback and CO₂
See how much you save on gas each month, each year and over the years you keep the car when you switch to a more fuel-efficient vehicle: fuel saved, gallons per 100 miles, CO₂ avoided and how fast the fuel savings repay a higher price. Works in mpg, km/L or UK mpg.
How it works
Yearly fuel cost = miles per year ÷ mpg × price per gallon · Yearly savings = (miles ÷ current mpg − miles ÷ new mpg) × price · Fuel use per 100 miles = 100 ÷ mpg · CO₂ = gallons saved × 8,887 g (gasoline) · Payback = price difference ÷ yearly savings
Each car’s yearly fuel bill is the distance you drive divided by its fuel economy (the gallons it burns) times the price of a gallon. The difference between the two bills is your saving, shown per year and per month, and the bills are repeated for every year you plan to keep the car, with an optional yearly fuel price change. Because mpg is distance per gallon, the gallons you burn fall steeply at low mpg and only slowly at high mpg. Going from 20 to 25 mpg saves more fuel than going from 40 to 50 mpg, which is why the EPA and fueleconomy.gov also show fuel use per 100 miles. The calculator shows both cars in that measure too. Under More options you can enter how much more the efficient car costs, to see the payback time, and pick gasoline or diesel for the CO₂ figure (EPA: 8,887 grams of CO₂ per gallon of gasoline, 10,180 grams per gallon of diesel). Fuel economy can be entered in US mpg, km/L or UK mpg and the price per gallon or per litre. The comparison covers fuel only, not purchase, insurance or maintenance costs.
Worked example
You drive 12,000 miles a year and swap a 22 mpg car for a 32 mpg car with gas at $4.29 a gallon. The current car burns 12,000 ÷ 22 = 545.5 gallons, or $2,340.00 a year. The new car burns 12,000 ÷ 32 = 375 gallons, or $1,608.75. You save 170.5 gallons and $731.25 a year (about $60.94 a month), or $3,656.25 over 5 years. Fuel use drops from 4.55 to 3.13 gallons per 100 miles, and the 170.5 gallons not burned avoid about 1,515 kg (3,340 lb) of CO₂ a year.
Sources: EPA: MPG and Your Wallet (gallons and cost at 20 vs 50 mpg) · EPA: Greenhouse gas emissions from a typical passenger vehicle (8,887 g CO₂ per gallon) · CarMax MPG calculator (two-car fuel cost comparison) · AAA gas prices: national average (Oct 2026) · EIA: Gasoline and diesel fuel update
Frequently asked questions
How much do I save going from 20 to 30 mpg?
At 12,000 miles a year you burn 600 gallons at 20 mpg and 400 gallons at 30 mpg, so you save 200 gallons. Multiply by your gas price: at $4.29 a gallon that is $858 a year. Enter your own mileage and price above for your figure.
Why does a small mpg gain save so much on a low-mpg car?
Fuel used is miles ÷ mpg, so each extra mile per gallon removes many gallons when mpg is low and few when it is high. The EPA shows that dropping from 20 to 19 mpg costs about 39 extra gallons a year at 15,000 miles, while dropping from 50 to 49 mpg costs only about 6. This is often called the mpg illusion.
Is it worth buying a more fuel-efficient car to save on gas?
Compare the price difference with the yearly fuel savings: enter the extra price under More options to see the payback time. If the payback is longer than you will keep the car, the switch does not pay for itself on fuel alone. Selling a paid-off car to buy a new one rarely pays back from fuel savings because of depreciation.
How much CO₂ does saving a gallon of gas avoid?
The EPA puts the CO₂ from burning one gallon of gasoline at 8,887 grams (about 19.6 lb) and one gallon of diesel at 10,180 grams. The calculator multiplies the gallons you save by that figure.
Should I use city, highway or combined mpg?
Use the combined rating unless your driving is mostly one kind. The EPA’s combined figure weights 55% city and 45% highway driving. Your own average from fill-ups (miles driven ÷ gallons added) is the most accurate input of all.
Can I enter km/L or L/100 km?
Pick km/L or UK mpg in the unit menu next to each fuel economy field. For L/100 km, convert first: km/L = 100 ÷ L/100 km, so 7.5 L/100 km is 13.3 km/L.
Related calculators
Educational estimate only: real fuel use depends on your driving, and prices change. Not financial advice.