Apparently i dont understand the 3% rule either. Please help.

Calcuations were done by the miata tire size calculator.

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

I am trying to decide between:

255/40/18 (-0.2% vs stock size)
285/40/18 (-1.1% vs stock size)

or

265/35/18 (+2.6% vs stock size)
295/35/18 (+2.1% vs stock size)

i am trying to keep the width within 30mm from front the back and minimize the difference from stock in overall height.

Thanks.

 

If your front wheel was 9.5" instead of 9" I would go with the 265 and 295 setup but the 265 might look odd on a 9" wheel, allthough safe, as long as your offsets are ok for those sizes.

What would look good is 255/40 and 305/35 but your rear offset would have to be spot on.

On the other hand the 255 and 305 might not agree with your Z, unless you have a base model.

Your best bet would be the first choice you mentioned.

 

I have VDC, so that is why i was trying to keep it within 30mm.

Wheel offsets are

F 18x9 +31 with a 10mm spacer = +21
R 18x10.5 +43 with a 20mm spacer = +23

 

Hmmm since you're using spacers, you could actually run a 305 on the rear if you switched to a 10mm spacer. But trust me trying to run a 305 is kind of a hassle.

I would still say go with your first choice. With the 285 rear if you're fenders are not rolled and you're dropped, you 'might' need to cut back to a 15mm spacer or roll the fenders a little but I'm not sure, I think 23 would be ok but could be close to hitting the fender lip.

As far as the 255 and 285 sizes your VDC will be fine.

 

I would do the 265 295 combo or throw 275/35 fronts with the 295/35 rears. 255/40 and 275/40 combo will also work just fine, you want to change the widths equally front to rear.