Wacky fuel guage

[Deutscher]

Fuel guage had just hit reserve and I stopped and filled up. Put the expected amount of petrol in.

On driving off I noticed that the guage was only registering half-full.

How can a guage start mis-reading like this, and is there an easy way to fix it?

Thanks for any advice. (Car is E46 M3.)

[Schermerhorn]

The measuring device in the fuel tank might be dirty. Stick some fuel injector cleaner in there and give it a good run. Only costs a fiver and will probably sort the problem out. If not, you've only lost a fiver. Worth trying.