Im going to have to let mine go right down to zero, if possible. See what i actually get in for sure. Im guessing around 45litres. After reading the manual in more detail, it turns out this 7 litre reserve means there is 7 litres left when the needle first hits the red. Which my computer shows as about 50miles. So it looks like the tank is 50 litres but, depending on the individual car, around about 5 litres of that is not being read by the sensor/ range computer.So for economy figures, we will be taking the tank as a 45litre tank.

Drove about 10 miles with range showing 0 miles, fuelled up to first click at 45.5 litres.

It would seem to change depending on car. Suppose different cars have the sensors set up differently

I just filled up this evening, travelled over 15 miles with the computer reading zero, it took 45.75 litres. I had done 555 miles which averages 55.1mpg, not bad on winter fuel! So if there is 50 litres in there, I recon I could have gone another 50 miles before emptying the tank completely!

Another one that filled up this evening! Trip computer said 10miles left in tank, Had completed 455 miles (most was start stop stuff, with a couple of hundred motorway in the middle.) Anyway, tank took about 41 litres (to the first click, as apparently anymore and you are taking up the space left for expansion). Lets say 42 to 43 if id run it down to zero. So around about 7 litres left in when the range says zero.

As for MPG. If we are saying 9.5 gallons useable until the range says zero, and 465 miles on the tank, thats 49 MPG. About 10 less than the stop start figure quoted. Lets hope that improves!