Yep, it's hilariously pessimistic. You have to remember the tank isn't a nice regular cuboid or cylinder, so the reading won't be linear anyway (I can drive 50 miles from full with the gauge hardly moving, but the next 50 miles move 1/4 of the gauge). When you hit the red there are about 2 gallons left I think (9ish litres).
So mine will go about 60 or 80 miles in the red, but I tend not to risk it and fill up a lot eariler anyway. The tank's meant to hold about 30 litres, so next time you fill up subtract what you put in from 30 and you'll have a rough idea how much was left before you started. Most I've put in was 29.6 litres(!).
The Metro/100 never holds 35 lites of fuel. I used to drive them when they wear new at work but we could only get in 30 lites before it would spill out. The fuel gauge has two know problems,
1 - Do not open the petrol filler unless you re-filling. The reason for this is simple the breather valve opens up to let air out of the tank so it can be filled to the top but if you don't fill it then the valve stays open and hence you will notice the gauge drop. When you take it off to fill it up you should get a sucking noise which mens the pipes and valves are in good nick,
2 - Only on later models when the gauge stays where it is when the ignition is off. The sender unit is supposed to be on a stabilised voltage to keep a good reading but the earth on the sender is pretty poor and you get the problem that 111GSI said it drops after so many miles when it should drop as soon as you drive.
Neither is a major problem and should never cause any real problems.
Yep I have to say the fuel gauge is pretty non-linear at times, but hey at least it works because that imitation fuel tank just ain't all that big really.
Since we're talking about the R100 / Metty fuel tanks, does anybody know if the R100 fuel tank has any restrictions/baffels/anti-theft devices in the filler neck fuel line into the tank??
The reason I ask is because sometimes it will back-up when refuelling and you get the inevitable 'click' as the pump cuts out. Also I have noticed a peculiarity when if you are filling the tank, when its filling up to half way you will get a sort of frothing up and a backing up of the fuel causing it to click. Once that has subsided it will just continue as normal - except if it clicks when its full or if it is backing up.