This may be common knowledge but it certainly caught me by surprise!
I was off to France where fuel is cheaper, on the way down to the ferry I realised that I was getting close to empty so I stopped to put in a couple of gallons to tide me over. When I got back in the car, the fuel gauge and miles to empty showed exactly the same.
Initially I panicked, thinking the gauge had failed or there must have been something wrong with the fuel pump I just used. However a quick Google on the iPhone revealed that due to the design of the tank and the position of the pick-up, when the amount of fuel drops below a certain threshold in the tank, the car estimates the amount of remaining fuel in the tank based on consumption. This is because it can no-longer measure it. If you do not put enough fuel in the car to allow the pick-up to register that the tank has been topped up, then the car continues obliviously and shows that you are running on fumes - most disconcerting!
When I got to France, I filled the tank and all was well - phew!
I thought this was interesting and worth sharing.