18 gallons is right according to Chiltons (and the manual). I agree with not letting it drop much below 5 gallons though, on the strip or in the "twisties".
87gnNV, do you have a digital dashboard? I ask because I have the same, and my "turbo lights" don't work either. It doesn't bother me, because I have a VDO boost gauge. But are the digital dash cars the only ones with the "lights" (yellow when turbo is "on", and red when over 10 psi I believe)?
I only ask because if you have a digital dash car, therefore a digital fuel gauge, Don't trust them! I've seen mine bounce around between 4 and 8 gallons throughout a small trip. I've run out of gas on "9 gallons" before too. Just warning you, they're sometimes uppity!