I have a Jaguar XJ8 4.0 LWB Sovereign. I wondered what the UK spec MPG consumption is meant to be ?. Different web sites have different figures. Its just when I first got the car it seemed excellent on fuel on the motorway. But I put it on my diagnostic machine and cleared some error codes and not sure if this is linked to it seeming like it is now not so good on fuel. Also is there any way I can tell when the air con is turned on ?. When I press the air con button it beeps but the red light on the button does not work.

Many Thanks
Mat

 

I have exactly the same model as you, mine is 1998. Over 120,000 miles the fuel consumption with the air conditioning on all the time is 23.1 mpg.

 

Maybe use OBD to check:
1. if it's running closed or open loop
2. fuel trims, if CL

 

If the light does not stay on, the system is off. Most likely low on Freon.