How to split the cost of a car nobody quite owns
Two people, one car, and no agreement about money. It works fine for about four months. This is how to set it up so it keeps working.
Start with what the car actually costs
Almost every argument about a shared car starts from the wrong number. One person is thinking about fuel, because fuel is the bit you physically pay for at a pump. The other is thinking about the insurance renewal that landed last week.
The real figure is everything the car costs over a year β fuel or charging, insurance, road tax, servicing, tyres, and depreciation β divided by the distance it covers. Depreciation is usually the largest line and the one nobody counts, because no invoice ever arrives for it. Roughly, it is what you would lose selling the car a year from now.
Work yours out first, because every method below depends on it: the car sharing cost calculator
Method 1 β pay per distance
Everyone pays a set rate for every mile or kilometre they personally drive. It is the fairest method by some distance, and the only one that survives people using the car very differently.
It also handles the awkward cases automatically. Someone who borrows the car twice a month pays for twice a month. Someone who takes it on a 900 km trip pays for the 900 km. Nobody has to argue that this month was unusual.
The catch is bookkeeping. Per-distance splitting means someone writes down odometer readings, and that someone gets quietly resentful about three months in. This is the failure mode to design around, not the maths.
Method 2 β split the standing costs, pay your own fuel
Insurance, tax and servicing get divided evenly, and everyone buys their own fuel. Simple, and it needs no records at all.
It is fair only when everyone drives roughly the same amount. If one person does 20,000 km a year and the other does 3,000, the second is subsidising the first on tyres, servicing and depreciation β all of which are driven by distance, not by time.
Reasonable choice for a couple who share everything anyway. A poor one between friends, flatmates, or family members with different circumstances, precisely because the unfairness is invisible until someone does the sums.
Method 3 β a flat monthly amount
One person owns the car, the other pays a fixed sum each month. No admin, no readings, no calculations.
This works when the usage is genuinely predictable and both people accept it will not be exact. It stops working the month someone barely drives, or the month the clutch goes.
If you use it, set the amount from the per-distance figure and expected usage rather than picking a round number that feels about right. And agree in advance what happens when a big repair lands, because that is the conversation that ends arrangements.
What actually goes wrong
Almost none of these arrangements collapse over the method. They collapse over record-keeping.
Someone forgets to note a reading. Someone remembers a trip differently. Payments happen irregularly, so nobody is sure what is outstanding. Six months in, one person is doing all the admin and quietly keeping score, and the arrangement has become a grievance rather than a system.
The fix is not a better spreadsheet. It is making the record something both people can see, that neither has to maintain, and that shows who logged what β so a disagreement is settled by looking rather than by remembering.
Agree these four things up front
Whichever method you pick, decide these before the car is shared, not after the first disagreement: the rate or amount and when you would revisit it; who pays for tyres, servicing and repairs; what happens if someone damages the car; and how often you settle up.
Settling monthly is usually right. Long enough that it is not constant admin, short enough that nobody is carrying a large unpaid balance they have started to resent.
Questions
What rate should I charge per mile or kilometre?
Work it out from your own car rather than copying a number. Total yearly running costs divided by yearly distance gives a figure you can defend, and it will differ a lot between an old diesel and a new EV.
Should I include depreciation?
Yes, if the car is being used significantly by someone else. It is usually the biggest single cost of running a car and it is caused by distance and age. Leaving it out means the owner absorbs it invisibly.
What about insurance for other drivers?
Check the policy before anything else. Named-driver and any-driver cover differ enormously in price, and an informal arrangement is worth nothing if a claim gets refused. This is the one item worth sorting properly.
How often should we settle up?
Monthly suits most arrangements. Weekly becomes a chore; quarterly lets balances grow large enough to cause friction when they finally land.
Is a written agreement necessary between family?
A formal contract, rarely. A written note of the rate, who pays for what, and how you settle, almost always β not for enforcement, but so that six months later you are both remembering the same arrangement.