Initial install of the JBD will reflect incorrect MPG until the car becomes accustomed to what the JBD is doing. Give it some time to settle, and then you'll get an accurate readout. The best way to determine your actual gas mileage is to track from tank to tank.
'09 E93 DCT M3 - Jerez on Ext Blk Nov w/Carbon Leather Trim
Custom Tune, OEM Perf Mod, MS Stg 2 Intake, GTS DCT/Euro MDM - MR Hood Vents/sidemarkers, H8 160, tinted tails, Gloss black kidneys w/metallic M fins, V1
WTB: E93 LCI Tails w/harness - Driver heated mirror - Exhaust tips - Driver Door Panel