Can you use a 235/45/17 instead of a 235/50/17 on the rears of the E model with out any problems? Could you also put the 235/45/17 on the fronts instead of the 225/50/17? The reason I ask is I may have a deal on a set of slightly used Blizzaks but all 4 are 235/45/17 .

 

Check out this calculator to determine your speedo error. Very Handy.
http://www.miata.net/garage/tirecalc.html

Your new tires will result in a speedo that reads 3.5% too fast.