Fuel Tank Calibration

[Lostfreak]

I have noticed something strange about the fuel tank on my 1998 VT S Pack. As a lot of people would know, if you hold down the UP button and MENU when you start the car it brings up the second range of displays, such as Fuel L/hour, battery Volts etc.
I have noticed that if I go and put petrol at the car with 20L in the tank (according to digital read-out) and put in 30L it ends up saying there's about 60L in the tank instead of 50, and it always does this - it always says there is a lot more petrol in the tank than there really is. This also results in the Distance To Empty feature reading way too high.
Also when filled up completely the read-out will say 75L for the first 70km on the highway before dropping to 74.5! Then other times the numbers will drop like a rock within a few minutes.
Is there any way to change the calibration as it is obviously out by a fair bit?
Also are all VTs equipped with 75L tank, as I filled mine when it got down to a quarter tank and I could only fit 48L before it could take no more.

[jules]

i'm afraid the fuel indicators on commodores are well known to be rubbish. i think dealers were offering to re-flash them at one point - i'm not sure.