If you want to run the widest tire you safely can on your rim, I would go 245/45 up front and 275/40 out back. This would be perfect as far as keeping close to OEM tire height (both new tires would only be .2" shorter than stock) and staying within the tire manufacturer specifications for the size of the OEM wheels.

As far as going down one size -- you can run anything within the tire manufacturers specification for your size wheel and be fine. I think the base 370s are under-tired, so it's probably best to run as wide as you can.

yah, just keep an eye on the load rating and you're fine.

but going back to snows, it's actually better to run narrower tires than wide... so 255 would be widest i would consider on these cars (i dont plan on driving my Z in snow though, will be running 225/45 and 245/45/18's on my G this winter.).

Yeah, for a winter setup I would stick with the OEM sizing.

I just received my Blizzak LM-60s from Tirerack.com (sponsor!). I mounted them on 18" wheels so I can keep my sport package wheels with the same tires on and just switch out the entire wheels.

If you get all four wheels and tires the same size, you will be able to rotate them when the time comes. Winter wheels have more tread wear so this could be important if you are trying to squeeze value out of them.

If you get a more narrow tire, they are actually better than wider tires in the snow. Wider tires create more surface to push the snow and you effectively have little plows = not good. Narrow is better for performance.

Z drivers have had great success with Blizzak LM-60 and Michelin X-Ice 2 tires, both available for your 18" OEM wheels.

Narrow and square setup = better performance.
Wide and staggered setup = better looks.

I went with better performance..because... I don't want to crash. Simple as that.

http://www.the370z.com/members/eazyd...per-wheels.jpg