Fuel Gauge Question
 

My new to me 99 Boxster will only take 12 gallons when the fuel gauge reads empty., and then it reads just a hair under the full mark. I assume the gauge is a little off, but I don't want to push it too much and run out of gas. I've read the capacity is either 16 or 17 gallons, depending on the source. How much do you guys typically put in when you are empty or close to empty?
Thanks.

The capacity of the tank is 17 gallons.

Yeah the most I've ever gotten in the tank has been around 15 gallons, and thats when the needle is at or just below empty. It's annoying because I'd rather have a fuel tank that just tells me exactly what I have instead of being conservative but it is what it is.

For me the gas pumps always shut off too early, probably due to very poor venting of air as I fill up. I know its not wise to fill up too much since that can negatively affect the system but I do try to listen for the trapped air "burp" noises coming from the filler neck.

Unless I add at least another gallon after the pump automatically shuts off, the fuel gauge always reads a "hair" less than full. BTW I get 27-28 mpg (premium fuel/98'Boxster/5-speed/97,000 miles)