I filled the car (STi8 non-PPP) up this evening on Optimax. As I don't have a Shell garage convenient to any of my normal routes to work or customers, and my car LOVES Optimax, I filled it to the brim - by which I mean I squeezed in every last drop I could.

I didn't check the gauge as I left, but after about 10 miles I noticed the fuel gauge only showed half a tank, then it worked its way south as I drove on for the next 80-odd miles. Then, when it had reached just over a quarter, it started to work its way north again until it reached the likely-accurate position of three-quarters of a tank.

Does anyone out there know the configuration of the sender? Is it in a side-tube which might have got airlocked by overfilling, or did I do something nasty to the sender?

(I did this overfilling exercise on my old 328i once and half-way home the needle plummeted to zero - a very expensive new combined pump-and-fuel-gauge-sender was my punishment - the original being probably damaged by the pressure buffetting from the fuel hose).

Fuel gauge problems on these cars aren't uncommon. Search on here for "fuel gauge" to see if you can get any info, but when this happens it seems to be a dealer fix, so probably best to drop into them ASAP.

Mine did the exact same thing a few weeks ago! Filled right up, then noticed the gauge was stuck on exactly half a tank!.Stayed like that til I'd done about 40 miles, then started to read correct again. Hasn't done it since! ;) Don't worry too much about it!

just fill it every 200 miles fook the gauge :D