Quattro Fuel tanks

I was recently curious as to how far my car would travel when the fuel guage
and digi readout showed 0 miles left, I was concered after noticing the
guage went down quite fast by putting in smaller amounts of fuel example
£15.00 roughly 3 gallons.

So I filled up and let the car bleep as it does at approx 60 miles left and
the needle was approaching the red line and carried on driving, I had a can
of fuel in the boot for safty sake and reset the mileometer when i hit 0 on
the digi readout.

This is where it gets interesting, normally I would expect a reserve of say
1 gallon max usually less from the point the needle shows red and the cars
shows 0, but my car is still driving fine 90 miles later, I will let it get
too 100 miles and see what happens, but was interested to hear from anyone
with the quattro twin tank design on a B6 1.8T or 3.0 A4, mine is a 2002
model.

It seems the guage is not really faulty as it may be reading low fuel from
one tank but the other tank is still supplying the fuel? I do not know...

Any thought?

BTW the guage and car are working perfect, and I am getting very healthy MPG
40mpg+ I was just curious.