Fuel Economy Comparison Calculator
Compare two vehicles on annual fuel cost, cost per mile and L/100 km, with the payback period on paying more for the more efficient one.
Real-world, not the sticker. Divide miles driven by gallons bought over a few tanks.
Going from 22 to 34 mpg saves $736 a year at these miles. Note that mpg is a deceptive scale — improving from 15 to 20 saves more fuel than improving from 30 to 40, because gallons per mile is the linear quantity and miles per gallon is its reciprocal.
What this calculator does
Two vehicles’ fuel economy, your annual mileage and the fuel price go in. It returns gallons consumed and annual cost for each, the saving, cost per mile both ways, the L/100 km equivalent, the payback period on any purchase premium for the efficient option, and the ten-year saving.
Two vehicles rather than one, because “what does my car cost in fuel” is a number you already feel. “Is the more efficient one worth it” is a decision.
The mpg trap
This is the most useful thing on the page and it is genuinely counter-intuitive.
Miles per gallon is a reciprocal, which means equal-looking improvements are not equal savings. Over 12,000 miles:
| Improvement | Gallons before | Gallons after | Saved |
|---|---|---|---|
| 15 → 20 mpg | 800 | 600 | 200 |
| 20 → 25 mpg | 600 | 480 | 120 |
| 30 → 40 mpg | 400 | 300 | 100 |
| 40 → 50 mpg | 300 | 240 | 60 |
Going from 15 to 20 — a 5 mpg gain — saves twice as much fuel as going from 30 to 40, a 10 mpg gain.
The practical consequence is large and widely missed. Replacing the worst vehicle in a household saves far more than upgrading the best one. Trading a 15 mpg truck for an 18 mpg truck beats trading a 35 mpg sedan for a 45 mpg hybrid, over the same miles.
This is exactly why most of the world reports L/100 km and why the US EPA now also publishes gallons per 100 miles. Those scales are linear: one unit means the same quantity of fuel wherever you sit on the scale. The calculator shows both so the distortion is visible.
Use your own numbers
The window sticker figure is measured on a standardised test cycle under controlled conditions, which is what makes it comparable between vehicles. It is not what you will get.
Real-world economy diverges with short trips (a cold engine is dramatically less efficient for the first several minutes), highway speed (aerodynamic drag rises with the square of velocity, so 80 mph costs far more than 65 for the same distance), cold weather, roof racks, tyre pressure, terrain and traffic.
Getting your real figure takes two fill-ups: fill up, reset the trip meter, drive normally, fill up again, and divide the miles by the gallons it took. Do not trust the dashboard readout — those are frequently optimistic by several percent.
Payback, and its limits
The payback calculation is straightforward: purchase premium divided by annual saving.
A $4,000 premium against an $850 annual saving pays back in about 4.7 years. Whether that is a good deal depends on things this calculator does not know.
How long you keep it. A five-year payback on a car you sell in three is not a payback.
Resale value. The efficient vehicle may retain more of the premium, which shortens the effective payback substantially — and this is frequently the larger effect. Hybrids in particular have historically held value well.
Fuel price risk. The saving scales directly with fuel price. At $5 a gallon the same comparison pays back in three years; at $2.50 it takes nine. Buying efficiency is partly buying insurance against fuel prices, and the value of that depends on how much price volatility bothers you.
Financing. If the premium is borrowed, the interest belongs in the calculation. The auto loan calculator covers what a premium actually costs when financed.
Fuel is not the main cost of a car
An honest caveat, because it is easy to over-optimise this one line.
For a newer vehicle, depreciation is usually the largest single cost of ownership — frequently more than fuel, insurance and maintenance combined in the first few years. A more efficient car that costs $6,000 more and depreciates proportionally does not save money regardless of what it does to the fuel bill.
Fuel dominates in exactly one case: high mileage in an older, paid-off vehicle, where depreciation has already largely happened. Someone driving 30,000 miles a year in a paid-off 18 mpg vehicle is spending real money on fuel and efficiency is genuinely worth chasing.
The commute cost calculator covers the full picture including time, and the car affordability calculator covers total ownership cost against income.
The cheapest efficiency improvements
Before buying anything, the free and near-free options are worth listing because they are real:
- Tyre pressure. Under-inflated tyres cost a measurable percentage. Check monthly; it takes two minutes.
- Remove the roof rack when not in use. Aerodynamic drag at highway speed is a large penalty for an empty rack.
- Reduce highway speed. Drag rises with the square of speed. Dropping from 80 to 70 mph is a substantial saving on a long trip and costs you a few minutes.
- Consolidate short trips. Cold starts are the least efficient driving there is, so one chained errand run beats four separate ones by a wide margin.
- Remove weight you are carrying around for no reason.
None of these require a purchase, and together they routinely exceed the gap between two similar vehicles.
What this leaves out
- Depreciation, insurance, maintenance and tyres.
- Electricity for EVs and plug-in hybrids, which needs cost per kWh and efficiency in miles per kWh rather than mpg.
- Fuel grade differences, where premium is required rather than optional.
- Incentives, rebates and tax credits on efficient vehicles.
- Fuel price changes over the holding period.
- Trip-level variation. The annual average hides a great deal.
For the whole cost of getting to work, including time and parking, see the commute cost calculator.
How this is calculated
gallons = annual miles ÷ mpg annual cost = gallons × price per gallon L/100 km = 235.2 ÷ mpg payback = purchase premium ÷ annual saving
Frequently asked questions
- How do I calculate annual fuel cost?
- Divide annual miles by miles per gallon to get gallons used, then multiply by the price per gallon. Thirteen thousand five hundred miles at 22 mpg is 614 gallons, which at $3.40 is about $2,087 a year. Use real-world mpg from your own fill-ups rather than the window sticker, which is measured under conditions you do not drive in.
- Why is mpg a misleading way to compare vehicles?
- Because it is a reciprocal. Going from 15 to 20 mpg saves far more fuel over the same distance than going from 30 to 40, despite being a smaller-looking improvement. Over 12,000 miles the first saves 200 gallons and the second only 100. This is why fuel consumption per distance — L/100 km, or gallons per 100 miles — is the more honest scale.
- How long does it take to pay back a more efficient car?
- Divide the extra purchase cost by the annual fuel saving. A $4,000 premium against an $850 annual saving takes about four and a half years. Whether that is worthwhile depends on how long you keep the vehicle and what the resale difference is, which usually matters more than the fuel.
- What is L/100 km and why do other countries use it?
- Litres consumed per hundred kilometres — fuel per distance rather than distance per fuel. It is linear, so a difference of one unit means the same amount of fuel wherever you are on the scale, unlike mpg where the same gap means wildly different things at 15 and at 45. It also makes trip costs directly proportional.
- Does fuel economy matter more than other running costs?
- Usually not, for a newer vehicle. Depreciation is typically the largest single cost of ownership, often exceeding fuel by a wide margin in the first years, and insurance, maintenance and tyres add up too. Fuel dominates only for high-mileage drivers in older, paid-off vehicles — which is precisely the case where efficiency is worth optimising.