Instant fuel gauge

The instant fuel gauge on my 97 3.2 has started to work in reverse. Eg put the foot down and it the gauge shows 0 fuel being used. Coast or sit on the freeway and the gauge shows anywhere between 85 - 99 ltd per hundred. What’s the fix? Hope someone can help. Thanks.

I think what you are seeing is kilometres per litre?

Thats correct.
It used to be the lower the number the lower the fuel used. The higher the number the more fuel used. Mines now reversed.
Ive had a few problems with fuel consumption lately. Eg 20-25 lts per 100 ks around 8mpg.
It was put on scanner and came up with a code 1621. I was told that this was the MAF sensor.
I replaced it with a reconditioned unit but the instant fuel is back to front.
Im not a mechanic but can do everyday stuff and like to fiddle with my car.
Its a 1997 Jaguar x300 3.2 Heritage model. Its done 226500 ks. I love the car but it drives me nuts sometimes.
I used an independent Jag mechanic but he seems to tell me things, that when i work it out the costs of which, i could by a new car.!!! I get the feeling hes taking me for a ride.
Hope you can give me some ideas on the problem so i can be a little more informed and maybe fix the issue myself.
Thanks for your interest.