Based on this tire/wheel calculator
http://www.miata.net/garage/tirecalc.html

what I have now is 205/55-15

215/40-17 is recommended with only 0.4% of my speedometer being too fast

I want 215/45-17 with 3.1% of my speedometer being too slow
because it will slightly increase the diameter of the overall tire in combination with Eibach sportlines (1.5" drop) I can have a smaller gap and fill my tire well a little more.

Would you go with the recommended tire size to get a slightly smaller overall tire diameter but have a more accurate speedometer or get a larger overall tire diameter and have your speedometer be 3.1% off

Will this effect my total mileage - odometer too?

every 10,000 miles your odometer will be off by -300.66 miles.

use this calc for more precise info >>>
http://www.c5-corvette.com/tirecalc.htm

personally I think the difference in appearance between a 40 and a 45 series tire is enough to warrant a slight odometer error. so I would go with the 215/45/R17

I got 205-45-17's on my car and the speedo isn't that off. I used a GPS to see how off i was to 70 and it wasn't that off. So I would say get t he 45 serries profile because it looks better, helps protect your rims, and it helps fill out the wheel well.
Bryan

I have 215/45/R17 on my car along with the Eibach Sportline.I think they look fine.If you go with 45`s you would have your wheel gap back again and therefore not the look you are looking for. Any way that`s just my .02. >>

CDNSLEEKA,

Can you post a pic or your car and a closeup of your rims and wheelwell in the member's rides section?