Fuel Savings Calculator
Compare two vehicles’ fuel costs and see what switching would save.
Please note: Estimates for planning only. Real fuel economy, depreciation, insurance and lease terms vary widely by vehicle, region and driving style. Confirm figures against a dealer quote or your own records before relying on them.
What the Fuel Savings Calculator does
Comparing two vehicles on fuel means converting both to a consumption basis — litres per 100 km — because miles per gallon is a reciprocal measure. That reciprocal is why improving a thirsty car by 5 mpg saves far more fuel than the same 5 mpg gain on an already efficient one.
Formula
Litres per year = Distance ÷ 100 × L/100kmAnnual saving = (Current litres − New litres) × Price per litrePayback = Extra vehicle cost ÷ Annual saving
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Distance driven per year | number | Yes | Accepts more than 0. |
| Distance unit | one of 2 options | Yes | — |
| Current vehicle economy | number | Yes | Accepts more than 0. |
| New vehicle economy | number | Yes | Accepts more than 0. |
| Fuel economy measured in | one of 4 options | Yes | — |
| Fuel price per unit | selected currency | Yes | Accepts more than 0. |
| Fuel priced per | one of 3 options | Yes | — |
| Extra cost of the new vehicle | selected currency | Optional | Accepts 0 or more. |
| Years to project | number | Yes | Accepts 1 or more, up to 20. |
| Fuel type | one of 4 options | Optional | Used for the CO₂ estimate. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Distance unit and Fuel economy measured in.
- Enter Distance driven per year, Current vehicle economy, New vehicle economy and Fuel price per unit and 1 more.
- Optionally add Extra cost of the new vehicle.
- Select Calculate.
Worked example
Swapping a 22 mpg car for a 35 mpg one, driving 12,000 miles a year with fuel at $3.50 a gallon.
- Distance
- 12,000 mi
- Current
- 22 mpg
- New
- 35 mpg
- Fuel
- $3.50/gal
545.5 gallons drops to 342.9 — a saving of 202.6 gallons, or $709 a year and $3,545 over five years.
Reading the result
- "Saving per mile" isolates the improvement rate independent of actual annual mileage — total saving scales directly from this figure if driving distance changes.
- The "annual saving at different fuel prices" table shows how the case for switching strengthens as fuel prices rise — a marginal payback at today's price can look quite different under a plausible future price.
Assumptions and limitations
- Both fuel economy figures are trusted as entered — if one is a rated figure and the other real-world, the comparison mixes two different kinds of number rather than comparing two vehicles fairly.
- Annual distance is assumed constant across the whole projection period, though driving patterns commonly shift over a 5+ year horizon.
Common mistakes
- Comparing mpg figures directly instead of actual fuel saved — the FAQ explains the same mpg gap means very different savings depending on where it sits on the scale.
- Judging payback purely on fuel savings while ignoring the notes' point that insurance, tax and maintenance may also differ between the two vehicles.
Frequently asked questions
Why does mpg mislead when comparing cars?
Because it is fuel economy, not consumption. Going 10→20 mpg halves fuel use; 40→50 mpg cuts it by only a fifth. Litres per 100 km avoids the trap by being linear.
Does a more efficient car always pay for itself?
Only if the fuel saving outruns the price premium within your ownership period. High mileage drivers reach payback far sooner.