[SFE92335]

Hi guys, recently I've been getting an avg mpg reading of 6-8mpg from my 335i. I currently have a JB+ and Britalman exhaust but I have been running this setup for almost a year now averaging 14-17mpg when driven mostly in city and pretty hard. The only change I have made was re-adjusting my exhaust to try and get my tips centered. I brought it to my friends shop yesterday and put it on the rack while revving it to see if there might be an exhaust leak but they didn't hear anything. I have checked the codes with my BavAuto scanner and am coming up with no codes either. I will try taking off the JB+ to see if it makes a difference but has anyone experienced this before? If I am idling and at a stop I start seeing the mileage fall to 6mpg but when I start moving it goes up to around 12-13 steadily. But after the next start-up it goes back to starting at 6mpg. Thanks in advance.