widest tire for factory wheels

A&Q about 350Z
Q:

03 touring model with 17" wheels.
what are widest tires which will fit these wheels SAFELY.
thanks and drive safe.
A:

bluztrk, i just upgraded to 275/40/18 rear, 235/40/18 front with toyo proxes t1-s tires on the ray's rims that come with the track. they're pretty good. smoother ride, and the launching is completely different... and better. i swear the car is faster now :)
anyway, there are significant changes to the handling, but they all seem positive. got stuck in a torrential downpour the other day and felt pretty firm on the road. they have a good tread pattern.
875+tax, including alignment on Hunter machine, to nissan specs which i cross checked. now that the toe has changed, i may run it back in based on nissan's new specification - will have to see.
these tires won't last 12K - not because of feathering, but because of softness. it's hot and sticky where i'm at, so i like my tires sticky too. i posted a few pics in an earlier post - you can see the tires on the rims there.
2003 350Z LeMans Sunset Track with Nav
No mods
A:

350znewbie,
In order to keep the same overall wheel diameter, I believe you need 275/40/18 rear and 255/40/18 front. I think your 235/40/18 front is a bit small. Did VDC give you any problems?
Heres a site that does the math for you http://www.eunos.com/tirecalc/
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