245/45 on stock rims
 

will this cause the speedo to report a higher mph?

Nope. No issues at all with running 245/45/16

Right, like DamonB says, no problem.

Try using this tire size calculator for "what if" scenarios.

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

Mark

Hey thanks for the link. That is some good info!