The 500 fuel system is designed to operate with a significant air space (around 5-6 litres) and under no circumstances should you ever try to fill it beyond the second cutoff of an automatic fuel pump. The car's handbook contains a clear and specific warning against doing this.
Attempting to dribble fuel into the tank after the pump's automatic cutoff can cause permanent damage to fuel system components, particularly the EVAP canister. These components are eye-wateringly expensive; the repair bill will be thousands, not hundreds, and almost certainly won't be covered by any kind of vehicle warranty.
There have been a few EVAP failures reported here; believe me when I say it's somewhere you don't want to go.
Maximum fuel capacity is 35 litres; if you manage to get more than this in from running on fumes, you've overfilled it. If you refuel when the low fuel warning light first illuminates, you should get around 27-28 litres in before the dispensing pump cuts off.
Unfortunately this 'feature' does make it hard to get an accurate true mpg figure between any two tankfuls - but if you average over ten tankfuls, it'll be close to what the car is actually doing.
If you absolutely must have more range, carry a container of fu
