Your 215 tires will result in the speedo registering approximately 6.7% slow; i.e at an indicated 60 mph you will be traveling 64 mph. So long as you have clearance, your decision to use the 215s should be a non-event. That said, run a measured mile check (or borrow a radar gun from a friendly cop) to check your speedo. Some MB speedos are inaccurate (commonly a bit fast), so your oversized tire just might make the speedo more accurate.

FYI: When you buy tires in the future, the 185/75 size is an exact size replacement for the original and hard to find or expensive to buy 175/80 size. The speedo registers normal and the overall size is so close that you cannot detect a visual difference.

230/8