C4S Fuel gauge / computer

[oldhatter]

My 996 is the latest in a long line of cars with pathetic range on a fill up (Departed Noble M12 and EVO VI being prime examples, as well as still owned Caterham). Because of this I find the range to empty function useful and normally brim the tank at each fill with Optimax. However last week I twice got caught out and had to splash & dash with normal unleaded. The first time I made do with 8 litres, but was surprised that the range to empty didn't seem to register the extra fuel. I put this down to a glitch, but later in the week got caught out again on the Welsh borders and decided I needed 20 litres to be safe.
This time it was obvious that not only did the range till empty not change, but the fuel gauge didn't register the extra fuel either. I must admit I found running apparently empty slightly distracting, and the issue didn't resolve itself until I found an Esso station with Super Plus and filled to the top again. Do they all do this? Should I read the Manual?
Steve

[oldhatter]

Thanks Lightweight, I'll get it looked at under warranty.
Steve

[oldhatter]

"i tend to get around 300-350 miles from the tank, hardly what I would call low,"
(sorry I don't know how to do that "Jamie said" thing)
Looks like I'll have to get the GT3 on order PDQ :-)
Consumption seems to me to be very dependent on type of mileage. The 4S is probably the best everyday car I've ever had (though I still smile when I think back to 968CS) and I use it every day. Certainly judging by the computer the key is not weight of right foot but type of driving. Commuting in traffic I guess range is around 200 - 240 miles, but on a decent run this increses to 280 - 300. By a decent run I mean "using" the cars abilities properly.
Steve