volvo 960 1997 fuel gauge does not measure accurately

[jimmy57]

No, There is a time delay when ignition is on. The gauge will take something like 4-5 minutes per quarter tank movement of dash gauge based on my ownership of two 960's (one is now departed...).
I am in Texas, the car MIGHT burn to the ground if fueled while running but the WILL be hot when I get back in the summer if I turn it off. I'd fuel, jump in and go and not worry about fuel level on gauge until some time later. It always read where it should 20 minutes or more later when I would do a periodic gauge scan.
Next time you notice it not registering when left running, turn off ignition then right back and start the engine and see if it comes up to where you expect it should be.