What a kilometer actually costs you, petrol against electric
Every fuel-versus-electric comparison you have read used somebody else's numbers. Here is the arithmetic with yours in it, including the four costs that are usually left out because they are inconvenient.
Ask what a kilometer costs and you will be given a number. Ask where the number came from and it usually turns out to be a national average economy figure, multiplied by a fuel price from whenever the article was written, describing a car nobody in the conversation owns.
Your own figure is not hard to get and it is the only one that is any use. It takes two fill-ups and about five minutes.
Measuring your real economy
Do not use the trip computer for this. It is useful for driving feedback and it is usually optimistic. Use the pump and the odometer, which cannot flatter you.
- Fill the tank until the nozzle clicks off. Note the odometer reading. Do not reset anything else.
- Drive normally. Not economically, normally. A tank driven carefully to produce a good number is measuring a journey you do not usually make.
- At the next fill, top up to the click again from the same pump if you can, and note the litres and the new odometer reading.
- Kilometers travelled ÷ litres added = your km per litre.
One tank is a sample, not a measurement. Three consecutive tanks give you a range, and the range is more honest than any single figure. If your summer and winter numbers differ noticeably, that is real. Air conditioning, denser cold air and longer warm-up periods all show up here.
The single most common mistake is comparing your carefully-driven tank against somebody else's ordinary one, and concluding your car is more efficient than it is.
The fuel cost per kilometer
With your own economy figure, the fuel cost is one division:
| What you need | Where it comes from |
|---|---|
| Your km per litre | The three-tank measurement above |
| Today's price per litre | The pump, on the day you are asking |
| Fuel cost per km | Price per litre ÷ km per litre |
Do this again whenever fuel prices move. The number is not a constant, and treating a figure you worked out six months ago as still true is how budgets drift.
The same arithmetic for an electric car
Electric cars invite a different mistake: quoting the manufacturer's consumption figure and the cheapest tariff, then comparing that against a petrol car's real-world economy at the full pump price. That is not a comparison, it is two different questions.
Measure the same way. Charge to a known point, drive normally, and take the energy the charger reports against the distance travelled to get your kWh per 100 km. Two things to be careful about:
- Use energy drawn from the wall, not energy delivered to the battery. Charging is not perfectly efficient, and the difference is real electricity that appears on your bill.
- Use your actual tariff, including any slab structure. If charging pushes your household consumption into a higher band, the marginal cost of that electricity is the band you land in, not the average rate on your bill.
Then: cost per km = (kWh per 100 km ÷ 100) × your rate per kWh. Compare that against the petrol figure you calculated the same way, on the same day, from the same kind of driving.
Where CNG sits
CNG is priced per kilogram rather than per litre, so the arithmetic is the same shape with different units: distance covered ÷ kilograms used, then rate ÷ that. Two things belong in the comparison and are usually left out.
The first is availability, because a fuel you cannot buy on the day you need it has a cost that does not appear in the per-kilometer figure. The second is the conversion itself, which is a capital cost to amortise and, on many cars, a boot you no longer fully have. Neither makes CNG a bad choice. Both belong in the sum.
The four costs comparisons leave out
Fuel is the cost people compare because it is the one they see weekly. It is rarely the largest.
- Depreciation. Usually the biggest single cost of running a car, and entirely invisible until you sell. A car losing value steadily is costing you money every kilometer and every month it sits.
- Consumables that wear by distance. Tyres, brake pads, discs, filters, oil. Take the cost of the item and divide by the distance it lasts. A set of tyres over its realistic life is a real per-kilometer number, and it is not small.
- Scheduled servicing. Annual cost divided by annual distance. This is where a car with a documented history helps you: you know what it has had and therefore what is next.
- Fixed costs. Insurance, token tax, parking. These do not vary with distance, which means the more you drive, the less each kilometer carries. A car driven very little has a high true cost per kilometer no matter how efficient it is.
Add the distance-based costs to your fuel figure to get a running cost per kilometer. Add the fixed costs divided by your annual distance to get the total. The gap between those two numbers is usually larger than people expect, and it is the gap that explains why a cheap-to-fuel car can still be expensive to own.
FROM THE PRODUCT
The measurement, kept for you
Log a fill-up and Motorspree works out the economy for that tank, keeps the running average, and prices a trip in your own currency at the rate for your market, from your car's real numbers rather than a national average.
Common questions
It is estimating fuel used from injector timing rather than measuring it, and small systematic errors accumulate. Most read optimistically. Use it to compare one drive against another, which it does well, and use the pump for anything you are going to put in a budget.
On fuel, usually. On total cost, not necessarily. A larger engine working lightly can be less stressed than a small one working hard, and consumables, insurance and depreciation all move independently of capacity. Work out the total rather than assuming from the engine size.
Three gives you a usable range. Six across different seasons gives you something you can plan with. Any single tank is dominated by what you happened to be doing that week, which is why one impressive tank is not evidence of anything.
Yes. You will sell eventually, or the car will reach the end of its life, and either way the value it lost was a cost you paid across the years you owned it. Leaving it out makes every car look cheaper than it is, and makes an expensive one look like a bargain.
Motorspree Editorial
We write about keeping a car properly, and we say what we can back up. Corrections and disagreements are welcome.