you have about 1 gal. reserve when the needle is on empty, old gauges tend to be off so don't trust it too far on empty. You could replace the sending unit if it really bothers you

 

My gas gauge is totaly whack. Before I installed my new gauge faces when it got down near E the light would come on but I could still get at least 20-30 miles and be fine(never actually ran out though so I don't know how far it would go.) After the install the needle will be on straight down E and the light won't come on for a while then I can still get around for a bit like that.

Someone else around here said that the filling neck in the tank might be too low so that when the gas gets up to the opening of the neck the pump stops but the tank isn't really full. So, that could be a problem too.

 

I'm not too sure of your location but your gallons may be Imperial gallons. The imperial gallon is slightly larger then the US gallon. For example 11.9 gallons (US) would equal 9.9 Imperial gallons. The conversion is to divide US gallons by 1.2 to get Imperial or vice versa. This may be the reason you have only been getting 8.5 gallons in.

 

I wish that was the reason for my fuel tank weirdness. But I'm in Hawaii with regular U.S. gallons. Makes sense though. The car is 10 years old, more likely its the fuel level sending unit thats showing its age.

Mahalo!