OEM Tire Size on the 17's 205 / 45 / 17

If you go to a 215 / 40 / 17 will that alter your speedo accuracy?

Sorry if the question sounds ignorant...

 

Not enough to matter.

For exact effect ...

http://www.miata.net/garage/tirecalc.html

 

I did that exact swap, and my speedo shows me going faster than I am. Enough to matter.
I have intend to get my GPS out of my other vehicle & see exactly how much my speedo is off, but haven't taken the time to do it yet.
There are a lot of variables though. How much my speedo was off to start with, tire preasure, and the amount off at low speeds compared to the amount off at high speeds.
Once I know how far off, I should be able to estimate my speed fairly acuratly.
Another bit of info....the shorter sidewall rides a bit rough, even comparing to runflats.

 

2% small diameter from HERE