James,
Everything I can find on the web says that the 1999 Boxster has a 64 liter gas tank, which is 16.9 gallons...
I have a MY2000 owners manual, and it says the same ( except I know that the 1999 is a 2.5 and the 2000 is a 2.7 so the tank might be different ).
Not trying to be difficult, just trying to figure out how you guys are getting 30 + mpg :
http://www.fueleconomy.gov/feg/noframes/14812.shtml
Nick