problems with fuel mileage questions on international forums is that responses often involve different metrics that cannot be easily compared against one another. so, i'll offer one that doesn't involve any metrics at all. here it is.

i remember when these cars were first offered, including the late XKs, the new E types (XKE in the U.S.) and their brethren, and IIRC the consensus opinion was that, across the board they all got bad fuel mileage. and i can personally attest to that, although i won't describe what it is or volumes used or distances covered.

 

I would argue the definition of "Bad fuel mileage". There always has to be something to compare it with and when our cars were built in the 1960s using an engine designed in 1943, 18 to 20 MPG was possibly considered average and it was a luxury car. Rivals slightly above the Jaguar such as the Bentleys S2 and Rolls with a 6.2 litre engine were only doing 10 to 12 and the 2.5 litre Triumphs and Rovers were only doing 25mpg.

Comparing this to my 2013 Jaguar XF Sportbrake 2.2 diesel which is doing 50 plus MPG is not fair.

Comparing the difference of one Mk2 against another Mk2 from the 1960s is a fairer way of establishing whether your car is tuned correctly or not but you are right that converting Imperial gallons to US gallons to Litres is a pain.