I switch between 18" for winter and 20" (214s) for the rest of the year. I dont see any significant degradation, many things depends on your driving style, you will loose those 2-3 mpg if you are driving above certain speeds

You are in San Diego, I presume hills are not uncommon, 214s will not affect your MPG, but hills, highway speed, and accelerations from stops will !

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His : 2013 X5 3.5d Deep Sea Blue/Black/Aluminum with ALL packages, 20" 214"s, running boards, Adaptive Drive, Spare Tire, BMW Apps
On Order : 2016 X3 3.5 MSport Silver/Mocha with DHP, CW, Prem, Tech, DA, Lighting

Hers : 2015 328xi MSport Silver/Red

Prior:
2008 X5 4.8
2011 X5 3.5d
2010 335 xDrive
2013 335 xDrive