can I replace all 205/55R16 tires with 205/50R16 for a bmw??

You can replace the tyres with lower profile but your speedo will show incorrect as the circumference will be less, if you were to replace with 17" rims you could go to 205/45 x 17

You should go to 215 size for that aspect ratio if memory serves me correct..check rear fender fit first. Good luck!

Here, real simple NO. Wrong load capacity let alone the diameter and width (control) issues

Paul & Russ are spot on about the speedo & fitment. Load rating varies by tire & manufacturer. And could very well be identical between sizes. 205/215 is width of the tire in mm. 55/50 would the profile of the tire, and is a % measurement based on width that applies to the height of the tire: rim to tread. 16 is of course the width of the rim in inches. (406.4mm) so: 205*.55 = 112.75 (stock wheel diameter of 631.9mm) 205*.50 = 102.5 (diameter -20.5 mm or 3.3% adj) 215*.50 = 107.5 (diameter -10.5 mm or 1.7% adj) You should be able to go +10 on the tire width safely as long as it does not rub. Keep in mind lower profile usually means you feel bumps more, and have reduced "tire roll". Going wider with lower profile will make your car slightly more prone to hydroplane. The plus side is you should get more grip in the dry. Smaller diameter wheels should make you car slightly faster at acceleration at lower speeds, with a lower top speed. Just know you are going slower than indicated (beyond the stock 8% BMW speedo) and you will rack up more miles and use more gas in the same gear to go the same distance with smaller diameter tires. Good info on tire size: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46

Sorry Sean Load ratings are a regulated item. As are speed ratings. Max inflation does cause a variance in load capacities. Your analysis of ratios is excellent but the proper answer is still NO. Oh wait semantically it is yes. Rather you should not. 215/50x16 should have the same load capabilities.

I stand corrected about standardized load ratings. Thank you for that. In all seriousness though, 1356 maxload vs. 1323 is not cause for concern. I do struggle understanding with your "width (control) issues" concept though...

You are correct that there would be little steering control issue with this swap but my experience would suggest the ride would be significantly altered. I wonder why he asked.. The 205/55 is usually much less expensive than the 50 series.