Came back from the shop. Fitted 255/40R18 on my stock rims. OEM was 245/45R18... What's the difference on mileage?

245 x .45 = 110.25
255 x .40 = 102.00

Difference of 7.75

http://www.miata.net/garage/tirecalc.html
(STOCK VS NEW Tire Size)

Speedo Speed = 60mph
Actual Speed = 58.5mph
2.4% Too Fast

 

Funny how the stock size for the 17's on a Z are 235/50-17 and the 18's are 245/45-18. That is 1.6% too slow reading 60mph is actually 61mph with the 18's

But me runnig 275/40-18 is right in line with the 18's (245/45-18) .1% too fast.

 

Is 275/40R18 the widest tire that can fit on the stock 06 rear rims 18x8.5? If so, does it require any fender rolling if the car is lowered 1.5"?

 

you are pushing it to the limit with 275/40 if you are lowered 1.5"...not sure if it requires fender rolling.
look ...mine is stock with 275/40/18

 

That looks great! Thanks for the pic. I have been looking all over to see what the widest tire size is I can put on the stock rear rims. I have not lowered the car as yet, I am planning to. What size do you have on the front?

 

i'm getting 245/40 for the front sometime this fall. Those rears are a couple of weeks old (general UHPs from tirerack).