330ci are you really using a lot of fuel?
Are you leaking fuel, this may be more likely, fuel pump seal will usually leak over 3/4 tank. Easy to pull rear seat again, pop access cover and see if you have a heavy fuel smell.
Could be a fuel sender mis-reading as well.
Suggest you read the first post in the 2nd link in my signature and pay special attention to the fuel pump gasket comments.
The read the link for the Hidden OBC Menu and you will be able to see both fuel level senders and see how much fuel is in each half of the fuel tank.
Then also check your total distance you are able to safely drive. Most E46 can get close to or slightly more than 300 miles per tank.