When buying new tires should I stick with the 225/45/18 size or would it be alright to get some 245/40/18? What are the effects going to be?

 

245/40/18 is fine. it's what i run.

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

 

Wider tires = More grip, lower MPG and more $$$. It all depends on what you use the car for. I track my car 4-6 times per year so I have track only wheels/tires and staggered (225/40 front, 245/40 rear) all seasons for the street. Staggered looks kind of nice but can cause the car to push in the turns.

 

Exactly. I lost about 1/2 MPG with my 245's. Some people complain of a loss of turn-in sharpness on the wider tires.

245's look better, too.

 

ok...thanks.