Right now I'm running 255/40/18's on all four's I believe. Is it okay if I run 245's instead on all fours or would you recommend larger tires on the rear? Thanks

 

it depends, what wheels? same wheels front and rear?

normally you dont want to go with a narrower tire, otherwise the rim is more prone to curb damage.

 

stock 123's

 

i'd stick with whatever the stock size is recommended.

 

You are running to wide of a tire. The stock 123 are 8 inches wide and a 245 is what is recomended and is what comes stock on bmw sport package with 123's (530i's )

255 is wayy to stretched on the wheel-you will get better cornering and performance if you use the proper size tire. I mean 255 on 8 inches is not terrible but IMO is the absolutely max of what size you would put on there

 

Check that - did you replace the stock 245/40x18s with 255's? If you did for certain your speedometer will be off slightly ... check here to determine how the dimensions compare to the stock set-up http://www.miata.net/garage/tirecalcold.html

 

+1 255 is too big for stock 18". And the meter is definitely off.

 

OH! That explains why my speedometer reads 63-64 when I'm going 60. The tire kingdom up by me has a special for "sonic" tires I believe? Something with an S. Anyways they're $92 each. Considering my bf's are like $208 this is quite the deal. The salesman told me that they're speed rated H not V which means anything over 120 is dangerous? He added that if the tires were made in this size, they must be safe enough to use on a car needing such size. Should I get these tires? I'm planning on upgrading my wheels when the right deal for 166's or 124's arises so what I have on my 123's isnt SUPER important.

 

OH! That explains why my speedometer reads 63-64 when I'm going 60. The tire kingdom up by me has a special for "sonic" tires I believe? Something with an S. Anyways they're $92 each. Considering my bf's are like $208 this is quite the deal. The salesman told me that they're speed rated H not V which means anything over 120 is dangerous? He added that if the tires were made in this size, they must be safe enough to use on a car needing such size. Should I get these tires? I'm planning on upgrading my wheels when the right deal for 166's or 124's arises so what I have on my 123's isnt SUPER important.

 

+2 I am running on stock 123 as well with 245/40/18. You are better off with 245 with stock 123 wheels. my 2 cents...