I have changed my front width wheel size on the car, but I'm a little bit concerned incase I have issues with different rolling diameters as I have a the Tiptronic 996 Turbo.
The new width is 8.5" compaired to 8" (rears are still same) but my new front tire size will have to be a 255/35/18 compaired to 225/40/18 does anyone know if this is ok to run alongside the stock rear size?

 

The 225's are 25.08 inches in diameter. The 255's are 25.02 inches in diameter. As long as you're not rubbing anything when you turn the wheel or compress the suspension, you should be fine.

 

Don't guess.

Know for sure.

Live with the following: http://www.miata.net/garage/tirecalc.html

 

My friend is running 255s up front on his 996 c4 w/8.5" fronts. The 255s fit w/no problem but I'm not sure if he rolled his fenders. What's your offset? Also, if you're really concerned, you could go with 245/35/18.

 

It's a bit dangerous to screw with guessing tire sizes especially with the newer traction control systems.

use the tire size calculator and go as big as you want but get it to match OEM diameter specs.

Wrong rear tire sizes can throw off gearing as well.

Do it right!

 

Not guessing here. The 996 was designed for a wheel diameter of 25".

Front:
225/40/18 = 25.1"
235/40/18 = 25.4"
245/35/18 = 24.8"
255/35/18 = 25.2"

Rear:
265/35/18 = 25.3"
295/30/18 = 25"
315/30/18 = 25.5"

If he goes with 245/35/18 or 255/35/18 he's good. The key is to keep the difference from stock to under 3%.
Go to this link to verify for yourself.

http://www.1010tires.com/TireSizeCalculator.asp

BTW my friend is running 255F/295R with no problems. He says that the extra width up front helps with the awd understeer. In addition, if you interested, check out the gt3 forum on rennlist. There are some guys running front widths as high as 285. Not saying that I'd do this, but I thought it was interesting. Hope this helps.

 

You are close to all of those but the tire calculator I listed will tell you exactly and give the differential.