The V8 fuel gauge can get unreliable.  Fortunately it is very simple to fix, requires minimal tools and about an hour of your time.

NOTE:  Have your petrol tank as empty as possible and have the car windows down so vapours don't collect inside the car.

You need:  8 mm spanner, 14 mm spanner, small screwdriver, medium Phillips head screwdriver, methylated spirits, clean rag, clean work surface and about an hour.