The calculator calculates fuel as miles divided by mpg, then times the pump price. 12,000 miles at 28 mpg and 3.20 per gallon is about 428.57 gallons and 1,371.43 for fuel. You then add service, insurance, tires, monthly parking times 12 (or a yearly parking amount if you choose that option), and annual tax. Example 1 totals 3,881.43, or 323.45 a month and 0.3235 per mile. Fuel is about 35.3% of that total. Insurance is about 30.9%.
Example 2 at 5,000 miles, 32 mpg, and 3.20 plus the fixed costs from that example totals 2,640 a year and 0.5280 per mile, because the fixed costs do not shrink with mileage. Example 3 at 25,000 miles, 24 mpg, and 3.20 plus higher fixed costs totals 6,163.33, with fuel at about 54.1% of the total and 0.2465 per mile.
The fields are annual mileage, fuel economy, price, service, insurance, tires, parking or wash, and registration or vehicle tax. Parking is monthly by default and the calculator multiplies it by 12; you can enter an amount that is already yearly. The other amounts are yearly. Auto insurance goes in as one yearly premium. This form does not price those policies.
The default is a US year: miles, mpg, and a US gallon (about 3.785 L), not a UK imperial gallon. UK readers can still enter miles and mpg; put MOT in service and Vehicle Excise Duty in the tax line. Metric labels are optional. A typed 12,000 is not rewritten as kilometers. A comma and a period in 3.20 parse the same way.
One trip is on the trip fuel cost page. Receipts divided by mileage are on cost per mile. The TCO calculator adds purchase, a payment, and value loss. There is no car price here. Blank mileage does not fill 12,000 from the example and does not calculate a yearly total. With mileage at zero, only the fixed costs remain, with no fuel.
Enter the example 1 numbers, click Calculate, and match 3,881.43. Then drop to 5,000 miles: 2,640. That is an estimate from the categories you enter, not a live pump feed.