The short answer
Add depreciation, insurance, tax, fuel, servicing, tyres and MOT, then divide by the miles you actually do. Depreciation is usually the biggest line and the one nobody counts.
- Depreciation: what it will be worth when you sell it, subtracted from what you paid, divided by the years you keep it. On a newer car this dwarfs everything else.
- Insurance and road tax, which are fixed and known.
- Fuel: your annual mileage divided by your real mpg, times the price per litre.
- Servicing, tyres, MOT and repairs, averaged over several years rather than taken from a good one.
- Divide the total by your annual mileage to get a cost per mile, which is the only figure that lets you compare cars honestly.
What will ruin it
Be realistic about mileage. People who do a low annual mileage frequently discover the fixed costs per mile are high enough that a car club, taxis or hiring for the occasional long trip would be cheaper.
Why it works
Fuel is the cost people watch because they pay it visibly every week. Depreciation is larger for most owners, invisible until the day you sell, and decided almost entirely by which car you bought and when.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.