Just be aware that changing the profile from stock is going to result in a different rolling diameter, meaning your speedometer will be off.

 

the speed will be off but maybe just a bit.

that is why you need to get as close to your stock wheels and tires OD.

imagine for awd 17 wheels to 18 wheels... no matter how close the OD are, they are still a bit different.

 

Yes, I thought about that.

I thought it was measured by the RPMs of the car, since I am not changing the Drive wheels OD, I do not think that it will affect the Speedometer

I am going to keep the rears 255/40/18, and bump the front to 245/40/18(if that will fit, 235/40/18 else)

 

if you are going to do that on the stock wheel, yes the tire will fit but i don't know it'll look right.

if you are going to track your IS then that's not a good idea, unless you get wider rim in the front then change to wider tire.

 

I was planning on getting wider tires as well. I am not going to do this for another few months, have to decide what wheels I want.

 

i suggest you wait. wait for the hyper rev magazine to come out here and they will show you a lot of after market car company and what rims and tire size they use.

 

What is this hyper Rev magazine?

 

Since the stock wheels in 18" have 2 different circumferences (25.1" and 26"), only one must be used for monitoring highway speed. I'd assume it's the front tire at 25.1".

Going to a 245/45/18 tire results in a 26.7" circumference and would give a speedometer error of +6.4%. This means that the speedometer indicates 70 MPH when you'd actually be traveling 74.5 MPH. Smaller tires would result in even less error.

Since most cars I've seen tested read about 2 to 5 MPH low on their speedometers, this should in reality make the speedometer reading more accurate and should not be of much concern. 10% accuracy is specified by law and most manufacturers set the speedometers to read low. Thus, when there is a +4.5 reading over the actual speed from a larger tire, combined with a speedometer reading -3 too low, the actual speed is +1.4 MPH over the speedometer display.

(See http://www.miata.net/garage/tirecalc.html for a nice on-line speed vs tire size calculator.)

I seem to recall while reading the Nav manual that there is a tire calibration. If there is, I don't know if this only applies to GPS navigation or feeds into the speedometer.