Some Boxster's have problems with the fuel gauge sticking. So when you fill up the tank is full, but the gauge gets stuck at 3/4 full. And the same thing can happen for low fuel. The gauge gets stuck at 1/4 tank, but their is less gas than that.
I have noticed that the OBC fuel gauge is based on your OBC average mileage. So if your OBC average mileage is very high (say from a lot of freeway driving), and then you go driving around town, the OBC may be giving you an estimated range based off your average MPG but not your actual MPG.
I'm glad you got it fixed and wasn't anything serious! My dad used to run out of gas all the time doing, "aerodynamic testing," in his 944.
