When you shop for a more efficient car, the question is not “how many more miles per gallon?” but “how many dollars a year?” Fuel use depends on gallons per mile, which is the inverse of mpg, so the same mpg improvement is worth much more on a thirsty vehicle than on an already frugal one. This fuel savings calculator turns the two mpg figures, your yearly mileage and the gas price into gallons, dollars and CO2 saved — and tells you how long it takes to earn back any extra price you pay for the efficient car.
How to use the fuel savings calculator
- Enter your yearly mileage and the gas price you pay.
- Enter your current mpg and the mpg of the car you are considering.
- Enter the years you will keep it and any extra price of the more efficient car.
- Read the gallons and dollars saved per year, the total over the ownership period, the payback time and the CO2 avoided.
For the cost of a single trip use the fuel cost calculator; to measure your real mpg, the gas mileage calculator; for an electric alternative, the EV fuel savings calculator.
Fuel savings formula
Gallons saved = miles ÷ old mpg − miles ÷ new mpg
Money saved = gallons saved × gas price
Payback (years) = extra price ÷ yearly saving
CO2 avoided = gallons saved × 8.887 kg
- 8.887 kg — CO2 released per US gallon of gasoline (US EPA)
- The saving is per year; multiply by the years you will keep the car for the total
Worked example: 22 mpg to 30 mpg
- Old car: 12,000 ÷ 22 = 545.5 gallons; new car: 12,000 ÷ 30 = 400 gallons.
- Saved: 145.5 gallons a year, worth 145.5 × $3.50 = $509.
- Over 5 years: $2,545.
- Payback on a $2,000 premium: 2,000 ÷ 509 = 3.9 years.
- CO2 avoided: 145.5 × 8.887 = 1,293 kg a year.
Where mpg gains are worth the most
Each row uses 12,000 miles and $3.50 a gallon. Values are computed with the formula above.
| Upgrade | mpg gain | Gallons saved per year | Saved per year at $3.50 |
|---|---|---|---|
| 15 → 20 mpg | +5 mpg | 200 gal | $700 |
| 20 → 25 mpg | +5 mpg | 120 gal | $420 |
| 25 → 30 mpg | +5 mpg | 80 gal | $280 |
| 30 → 40 mpg | +10 mpg | 100 gal | $350 |
| 40 → 50 mpg | +10 mpg | 60 gal | $210 |
| 50 → 60 mpg | +10 mpg | 40 gal | $140 |
The mpg illusion
The table shows why mpg can mislead. A 5-mpg gain from 15 to 20 saves 200 gallons a year here, while a 10-mpg gain from 50 to 60 saves only 33. The curve of fuel use versus mpg flattens quickly: most of the savings come from replacing the least efficient vehicles. That is why fuel-economy ratings in some countries use litres per 100 km, where equal steps always mean equal fuel saved — see the liters per 100 km calculator.
If you drive a lot, the same percentage improvement is worth more, because savings scale with miles. If you drive little, a premium for an efficient car may never be repaid. The payback figure helps: if it is longer than you plan to own the car, the efficient model is a choice for other reasons. Also remember that the savings are in today's gas price; if prices rise, the saving rises, and the reverse.
The result is an estimate from the figures you enter. Real mpg depends on driving style, season, load and tire pressure, so use your own measured value for the old car and a realistic real-world figure for the new one rather than the sticker alone. For a hybrid option see the hybrid vs gas calculator.
Sources
Fuel use as miles divided by mpg is the definition of fuel economy used by the US EPA and Department of Energy (fueleconomy.gov), which also explains why gallons per 100 miles is the better scale for comparing savings. The CO2 factor of 8,887 grams per gallon of gasoline is from the US EPA. Table values were computed from the formulas shown.
Frequently asked questions
How do I calculate fuel savings from better mpg?
Compute the gallons each car uses (miles ÷ mpg), subtract, and multiply by the gas price. Moving from 22 to 30 mpg over 12,000 miles saves 545 − 400 = 145 gallons.
Why do small mpg gains matter more at low mpg?
Fuel use is proportional to 1 ÷ mpg, not to mpg. Going from 15 to 20 mpg saves 100 gallons over 12,000 miles; going from 40 to 45 mpg saves only about 33 — the same 5 mpg, three times less fuel.
What is the payback on the extra price?
It is the extra purchase price divided by the yearly saving. Divide again by the years you will own the car to see whether the savings cover it.
How is the CO2 figure worked out?
Burning a gallon of gasoline releases about 8.887 kg of CO2 (US EPA), so the saved gallons are multiplied by that number.
Does it include maintenance or other costs?
No. It covers fuel only; insurance, maintenance and depreciation differ by car and are not included.