Midway is awesome. Right down the road from my work (for now). They are moving.

Find the tires you like and get a quote. Then take it to Discount tire and have them match or beat it.

 

This is a great calculator for that stuff.

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

 

Switching to 245/45/18s means your car will be traveling a little faster than the speedometer reads (just a little over 1% as compared to using the stock 235/45/18 tires, or about 1 mph at 60 mph.) As the tires wear, they will get closer to stock size and the amount of error will drop. Other than the price and availability, the 245/45/18s also give you about a quarter inch more tread touching the road. The 245/40/18s are another good option, but they start off over 2% too small and the error gets worse as they wear.