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 M3/Jerez Black on Extended Novillo Black w/Aluminum Brushed Trim
WTT 219M 18" Charcoal Staggered for 19" Staggered in Gunmetal, Hyperblack, or similar
Looking for: MS Stage 2 Intake Charge Kit, Metsterschaft GT2, VT1-550, E92 CF Trim, V1