Fuel Savings Calculator
Put in what each vehicle returns and how far you drive in a year. This gives the annual fuel cost of both and the gap between them.
Yearly saving
$654.55
- Vehicle A, per year
- $1,854.55
- Vehicle B, per year
- $1,200.00
- Over five years
- $3,272.73
- Fuel saved per year
- 193 gallons
- Vehicle B is the cheaper to fuel, by $654.55 a year.
- The gain from improving a thirsty vehicle is far larger than the same mpg gain on an efficient one, because fuel used is the reciprocal of mpg. Ten to fifteen saves twelve times what forty to forty-five does over the same distance.
- Fuel is not the whole cost of changing vehicle. Depreciation, insurance and servicing differ too, and on a newer car they usually swamp the fuel saving.
About comparing fuel costs
The question behind this page is usually whether a more efficient vehicle is worth changing to. Fuel is the part of that which is easy to quantify, and it is the part people most often estimate wrongly, because miles per gallon does not scale the way it looks like it should.
The saving from a given mpg improvement depends entirely on where you started. Moving from 22 to 34 mpg over 12,000 miles a year saves about 193 gallons. Moving from 40 to 52 — the same twelve mpg — saves only 69. The first change is worth nearly three times the second.
That is why the biggest saving in a two-car household is almost always in replacing the thirstiest vehicle, even when the newer one shows a smaller improvement on paper. It is also why an efficient car driven a long way matters more than an efficient car driven rarely.
Fuel is not the whole argument, and this page does not pretend it is. A newer vehicle usually costs more in depreciation and insurance than it saves in fuel, and that gap has to be set against the figure here before the change makes financial sense on its own terms.
What it works out
- Annual fuel cost for two vehicles side by side
- The yearly and five-year difference
- Fuel saved in gallons or litres
- Why the same mpg gain is worth different amounts
The formula
Saving = (Distance ÷ Economy A − Distance ÷ Economy B) × Price
Work out the fuel each vehicle uses over the year, take the difference, and price it.
Over 12,000 miles the 22 mpg vehicle uses 545.5 gallons and the 34 mpg one uses 352.9 — a difference of 192.5 gallons. At $3.40 that is $654.55 a year, or $3,272.73 over five years.
The reason the same mpg gain is worth different amounts is in the shape of that subtraction. Fuel used is distance divided by economy, so it falls away as a curve rather than a straight line. Twelve mpg added at the bottom of the scale removes a large slice of a large number; the same twelve at the top removes a small slice of a small one.
Going from 40 to 52 mpg over the same 12,000 miles saves 69 gallons against the 192.5 above — less than half, for an identical improvement on paper.
- Distance
- Miles or kilometres driven in a year. Check the difference between two services if you are not sure.
- Economy
- What each vehicle actually returns, rather than the brochure figure.
- Price
- Per gallon or per litre. A rise makes every one of these savings larger.
A worked example
12,000 miles a year, comparing a 22 mpg vehicle with a 34 mpg one at $3.40 a gallon.
That works out to $654.55 .
- Vehicle A, per year
- $1,854.55
- Vehicle B, per year
- $1,200.00
- Over five years
- $3,272.73
- Fuel saved per year
- 193 gallons
Questions
How much will I save with a more efficient car?
Over 12,000 miles a year, going from 22 to 34 mpg saves about 193 gallons — $654.55 at $3.40 a gallon, or $3,272.73 over five years. The saving scales directly with how far you drive.
Why does the same mpg improvement save different amounts?
Because fuel used is distance divided by economy, which is a curve rather than a line. Twelve mpg added at 22 saves 193 gallons a year over 12,000 miles; the same twelve added at 40 saves 69.
Which car should I replace first?
Usually the thirstiest one you drive a lot. A small improvement to a vehicle doing 15 mpg and high mileage is worth more than a large improvement to one doing 45 mpg and sitting on a driveway.
Does the fuel saving pay for a new car?
Rarely on its own. A saving of six or seven hundred a year is real money, but a newer vehicle typically loses far more than that in depreciation, and insurance usually rises too. Fuel is one line in the comparison, not the whole of it.
Should I compare in mpg or litres per 100km?
Either works here because the tool converts both. If you are doing it in your head, litres per 100km is safer — it is linear, so the difference between two figures is proportional to the fuel saved.
How do I find my annual mileage?
Take the odometer reading from two consecutive services or annual inspections and subtract. It is usually higher than people estimate.
Does a diesel still work out cheaper?
It depends on the price gap where you are and how far you drive. Diesels are more efficient per litre but the fuel often costs more, and the advantage needs high mileage to overcome a higher purchase price. Put both sets of figures in and compare directly.
What about hybrids?
Enter the economy the hybrid actually returns in the driving you do. Hybrids gain most in town, where the engine can stop, and least at steady motorway speed, so the advertised figure can be well off in either direction.