Originally Posted by Spike100

That (what you say) will work. Consider 275/40-18 with 22 offset for your rears.

--Spike

Originally Posted by Motormouth

if you search rim offset on google you can get an idea.

Concept
the simplest way to explain it is that the lower the positive (+) offset, the closer to the centerline of the rim barrel the hub(where the lug nuts are) mounts to the rim.

a zero offset would be the rim completely equal on each side of the hub. every stock car runs positive offset (the Z is +30 stock), which means the hub mounting point is farther towards the outside of the rim than the inside. As the offset lowers(+30,+25,+20, etc), the hub mounting point moves closer to the center of the rim barrel. when the hub starts being on the inside of the barrel centerline, that is negative offset and the number increases as it moves closer to the edge of the rim barrel (-5,-10,-15, etc); so the higher the positive or negative number, the more extreme the distance from the centerline.

Visualize
(the car is on the right hand side of the drawing, the '[' is the rim face, the '----' is the barrel of the rim)

zero offset (+0): hub mounts in the center of the rim

----
iii[
----

positive offset (+15): hub mounts on the outside of the centerline

----
ii[
----

negative offset (-15): hub mounts on the inside of the centerline

----
iiiiii[
----

the Z comes stock with a 18x8 +30 rim. you see where they sit. the higher the offset (+35 for example) the farther IN the rim will sit from the fender. offsets also matter with rim width (obviously).

Rim Width and How It Affects Actualized Offset
To compare different width rims with the same offset (to compare how they will look) I always added the MMs for the size difference like this, since rims are measured in inches, but offsets are in millimeters:

1 Inch = 25.4 Millimeters

So you divide 25.4 in half to get the offset difference (since offset is measured from the center) of a one inch rim width differene, which is: 12.7mm. Then you take that 12.7mm and add it or subtract it to whatever offset you have for the narrower/wider rim to see what a comparable offset would be (I round to 13mm for ease).

example:

18x8 +30 rim (stock) compares to a 18x9 +30 rim (Grand Touring stock rim size [F])

1 inch wider = the rim will sit 12.7mm farther out, so if you wanted the 8 inch rim to sit the same way the 9 inch does, you would need a +17 offset on the 8 inch rim.

*this is only to see how much farther OUT it will stick, inner fender clearance is not taken into account with this method.

Example of Fitment

A very standard rim fitment on the 350Z would be:

19x9.5 +22
19x10.5 +22

that sits fairly flush. most guys on this site run about a +20 offset. that will be semi-flush, not sticking out.

Summary

a +35 offset sucks, you will need spacers

Originally Posted by sam2944

Spike, what you think on 275/35 rears? This compare a little more to oem diameter......

Originally Posted by sam2944

Im planning to move to 18, my Z is 17. OEM wheels are 7.5" front and 8" rear. I will go with 8.5" front and 9.5" rear. Wheel manufactures suggest go with 20 offset. What do you think? I believe is a good offset due to I can make changes with spacers.......

Also for this wheel combo I also use mathematics, I think the ideal tires are 245/40 front and 265/40 rear. Tell me what you think, I like the tire a little out of the borders of the wheel, not entirely flush. Thanks

Originally Posted by Spike100

275/35 is a better size for 19" wheels. That size tire is a little short for 18" wheels.

Your original suggestion (265/40-18") would work fine, although this isn't a common tire size.

--Spike

Originally Posted by Concave

for 19's i would do 35. 40 would be beefy