Kizashi Club

I'm looking a replacing my stock 235/45-18's with 235/40-18's. From what I'm reading the only real difference will be about .5 in less in total diameter and the speedometer will be about 2mph off. That I can deal with and the tire will fit fine. But will it cause anything else? Anyone have any insight on this?

What's the reason for the change? You will lose a little bit of handling going to a thinner tire. The shop that changes it out should also adjust the speedometer for you too unless you want more miles showing than what the car actually has on it. But I will admit, my experience with this is very limited.

Ride will be a little harder do to the shorter tire heigth. Speed OD will be off. I thought about going wide not shorter but kept with the stock size.You can judge the speed better with a GPS unit it will show the true speed. Mine had always been a little slower tha the speedo reads by about 3-4 mph. Just upgraded the Dunlops (worn out at 23k) to Continental Extreme Contact DSW 235 45 18. What a difference in ride, handling and traction in wet weather. Had a heavy shower while working in Naples Fl this morning and the tires felt great. Can't say enough about these tires. The local Ford Dealer matched the price from Tire Rack so saved me alot of issues of having tires shipped and finding someone to M&B the tires.

go here

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

it should tell you all the info you need
the downside of going from 45 to 40 is the ride with be more harsh and bumpy, nothing else really

You'll have a bigger visible gap between the tire and wheel opening - not very attractive.

A ton more selection in -40 than -45 is about the only positive - just about everything will be a negative to going to a lower profile tire.

Note that your speedo is already slow, going to a smaller tire is going to make it even more off. I don't know if this small difference would have any effect on the tranny programming - since there's no gear changes, it might not be an issue anyways.