I knew that that statement will come as it comes always from somebody

but it is a difference if I need 20 liters of gas per 100km or 45 liters

.
There is no need to talk about any further, any engine got a fuel consuption range from / unitl, it depends on many factors what the mileage is finally.
An carbureted, hot camed, low static comp. engine with a high stall speed, 4.88 gears and a food that is down on the floor most of the time will need more than a fuel injected, properly tuned, mild camed, high comp. engine with a low stall speed, 3.08 gears and a driver who push the padel once in a while, even if they got the same displacement.
And I just want to be on the lower side of the range of an 540cid engines fuel consumption.
Hannes