Combined MPG sign of issues?

[Bmwtvboy]

Recently, these past 3 months I have noticed a decline in my mpg. I used to get a combined H/C of about 19 and always 21 on HWY.
Now I see 18 combined and the last couple of tanks 15-17 mpg range.
Using 91 octane. Regular oil changes 5K/w filter. Syn:5/30 No burning oil
issues. Still accelerates smoothly but lately some rough idle at start up.
Ideas of where to start looking? O2 sensors all four changed out at 160 , now214,800K Original top end engine components. Disa valve and CCV unit replaced around 160K.