I'm going to have to agree and disagree with ejw2011. Yes, changing the tire size from OE will change the speedometer. In this case, though, the change is tiny. So I think a 235/55R-17 should work fine.
So a friend of my uncle is offering me his winter rims and tires set. I wonder if 235/55R17 will fit my 2011 BMW X3 ? My X3 is 28i.
Just to make this thread useful to others, let me recap the F25 X3 original equipment sizes:
17": 225/60R-17 about 753 rev/mile (varying slightly with tire brand and model)
18": 245/40R-18 about 751 rev/mile
19": 245/45R-19 about 751 rev/mile and optional rear fitment 275/40R-19 751 rev/mile
20": 245/40R-20 about 750 rev/mile and optional rear fitment 275/35R-20 755 rev/mile
The 235/55R-17 proposed by the OP is about 766 rev/mile, so it varies from OE a bit more than the above range, but is still within 2%.