The sidewall thickness doesn't matter. It's the overall height that will change the speedo and tach if different from stock.
No you're saying sidewall height of a 205/55 vs say 225/55 sidewall height with both 17" wheel diameter makes no difference in overall diameter? That is basically what you're saying, which is factually wrong.
What happens if you want to go from say a 17" wheel to 18" wheel and want to maintain overall diameter? What changes is the sidewall thickness.
Sidewall thickness is part of the equation to determine overall height.
The math is:
2x Sidewall height (and converted to inches) plus wheel diameter.
2x (width x aspect ratio in decimals), then convert to inches plus wheel diameter. That gives you the rough diameter. Actual diameter will vary from tire to tire (design and production variances), especially since Michelin likes to give you less tread depth to start with.