Price per litre is not the monthly cost
Fuel cost is litres used multiplied by the price per litre. A car with high consumption can cost more even when petrol is cheap. Use litres per 100km and your actual monthly distance.
Octane and the handbook
Use the octane rating the manufacturer specifies. A higher octane is not automatically better, and a lower octane can cause knocking in an engine designed for premium.
Route and traffic
Short trips, idling and stop-start traffic increase consumption. The same car can use noticeably more fuel on a school run than on a highway commute.
Maintenance affects consumption
Tyre pressure, air filters, wheel alignment and a faulty sensor can change fuel use. A fuel-cost comparison that ignores maintenance is incomplete.
Keep the date and the local price
Fuel prices change. A price observed today is not a forecast. Keep the local currency and the date with the number.