Unleaded, Madam?

I went to get some petrol this morning, before driving up to Joyce's.

So far I've never seen the low-petrol warning light go off in my car, even though the hand in the gauge has dropped well below the lowest line. In most cars the hand drops to the lowest point when the engine is swithced off, but as this doesn't happen in my car, I don't know how low I can let it fall without refuelling.

The tank is 65 Litres, but the guys at the garage are able to fill the tank with much more petrol than I ever would. They put an automatic filler on while they wash the windows, and check the oil and water. After the automatic filler stops, they continue pushing another 10-15 litres in the tank by holding on to the nozzle themselves. Because of this habit, I never quite know how much petrol there actually was left in the tank before the car was filled up again.

I've tried to drive with the car for as long as I've dared, just to see if the light goes off, or to see how low the hand really drops. Now, that I was driving to the less than formal settlement, I decided to quit the experiment for now.

"Good morning Madam, how are you this morning?"
"Morning. Fine, and you?"
"Fine Madam, thank you. Will it be Unleaded for you, Madam?"
"Yes, please."
"Sorry, we're out of stock, Madam."

I wonder what's wrong with the petrol supplies this time. I did manage to fill the tank up at the next garage, but they had just a little bit left. They measured the petrol by sticking a long wooden pole in a hole in the ground, and I saw they were down to the last 5 cm or so. No idea what that is in litres or gallons, though, but I was happy I refuelled the car now and not an hour later.

I've been thinking of filling the tank myself to get a better idea of how much petrol really fits in the tank when the gauge's hand reaches the lowest line. But as I can't even put my own shopping in bags at the supermarket myself, I don't think it would be proper of me to fill my tank. That would perhaps be seen as stepping on someone's toes, or at least depriving someone of the R2-5 they would otherwise get for helping me.

P.S. We just managed with the 3Gb in January: total bandwidth =2.986Gb.