My Z4M best is 29 mpg drops to 26 around town.
If it remains a s bad as you say, it could possibly something as simple as a lamda sensor (O2 sensor) ..... injection systems switch to default mode which is rich when the sensors fail, however, I'd expect to get an emissions failure on the dashboard should this be the case.
If it remains a s bad as you say, it could possibly something as simple as a lamda sensor (O2 sensor) ..... injection systems switch to default mode which is rich when the sensors fail, however, I'd expect to get an emissions failure on the dashboard should this be the case.