Around town I would think you should be between 250-300miles to a tank approximately. Code readings are irrelevant if there are no codes and may tell you nothing at all. If it hasn't had the spark plug leads or coil pack changed in all its life then maybe it is worth changing those with a brand new set of spark plugs too and also a fuel filter if that hasn't been done. That should give you a good base to start from. I'd go to a garage that can read live data and check that lambda readings are ok and not under-performing.