You will NOT surive with summer-performance 20" Bridgestones or Dunlops - dont even bother, especially in LI... I once drove on them in 1" wintery mix/slush, and I felt like I had skis instead of tires (car was 4 mo old). The car was sliding sideways while waiting at an intersection on a slopy surface.

I do have 18" all-season bridgestones for winter driving, which also help dealing with potholes and cold temps/snow.

And since you are averaging about 10mpg now, I'd guess you are either heavy footed or have lots of stop-n-go....In either case, switching to 18" would not help mpg-wise

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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