Originally Posted by magne

If you use 45 profile with 18" wheels the diameter of the wheel will be bigger than stock. So the speedometer will show wrong speed, the odometer will be a bit off, and you may also be breaking the regulations about wheels in your state\country.

The site you use to calculate rim offset does not show a very good indication of how the tire fits.
It does however give a good idea of what altering offsett will do to your car when it comes to if things will fit or not.
The best you can do is to use this site and ruler to meassure if things fit to your car.

To explain a bit to you about tire sizes, if you are not aware of this:
225\40-18. This means that the tire is 225 millimeteres wide. 40 is the sidewall hight in percent of the width of the tire. 18 is the rim size.
If you alter the profile from 40 to 45 you'll get a bigger impact on how high the sidewall will be if the tire is wider. Going from 40 to 45 will make the sidewall 5% of 225millimeters higher. (about 12mm higher)

when it comes to winter tires, there are not many sizes in low profile. Therefor a widely used size is 225\40-18. Also 225\45-17 is a very common size.

From what you have explained about your needs and wants in the previous posts, you'll be fine with 18" rims and 225\40-18 allround the car. That way you can buy 4 stock 18x8" rims, they will fit fine, have correct offsett and tire size.
245\40-18 will fit the 18x8,5" rim. But I don't see the point in splashing out the extra cash for wider tyres in the winter. These tyres are narrower than stock tyres, but will still fit your rims. However youl'll lack some of the sidewall protection for your rim. Also 245\40 differs quite a bit from 255\35. So again you'll might throw off the speedometer and odometer.

If I were you I'd just go for 18x8" with 225\40-18 allround. This gives you a lot better choises of tyres and you can do it on stock rims.

I did go for 17x8" with 225\45-17 on my car. Just because we get a lot of snow and the road gets very bumpy were I live. 17" was a good fit on my car, and the 18" stock wheels are very expensive where I live, so this probably saved me about 5000$ compared to the stock 18"

I've linked you this calculator before, it is a very good one. But in a foreign language: www.motormann.no
It shows every vital messurement in both rims and tires, and it lets you compare your combo to the stock combo.
Try to use it, even if it is a foregin language, most of the info are pictures and numbers. And those are understandable i any language

Originally Posted by Bass Mech

sony, why not just buy some good all season tires on same size as stock? put those on your factory wheels and buy a summer set for the other rims?

look at the continental DWS that stands for Dry, Wet, Snow they are a very highly rated tire very popular here in colorado.

after doing a lot of research i have finally settled on
235/35/19 front and 275/30/19 rear because i have verified that both the GS and the IS read about 1 mph high anyway. a larger tire will fix that and will fit the LFA wheels better because the tire sized i mentioned are wider.

i would caution you about getting a 245 on the front i cannot recall but i think your getting the AWD yes? the 345 tires are more then 1% taller than the 275 in the rear will be.
the biggest issue is i know from my old 2IS the trac control is VERY sensitive, others have posted the 3IS even more so! if your rears are rolling faster than the fronts your just that much closer to activating the trac control when your not spinning.
I've seen pictures of the 235 on the front on the exact same wheels right here on CL look for poster CHOW he has a black 2IS with the exact same wheels and these same tire sizes the fronts are only slightly narrow but i think acceptable on the front. the rears look nice and square.

BTW yesterday i ordered my black LFA wheels from OEM wheels+ dot com $709 for the set shipped!
the local discount tire store and myself pout together a quote for just about 1,000 for the 4 wheels, 4 ez sensor TPMS mount, balance and tax.
I'm going with the Continental extreme contact DW they won't be good at all for snow, but this will not be a snow driven car for me anyway, i have an awesome 4X4 truck and a set of 4 studded snows with a set of chains for that.

Originally Posted by jaznraz

I hope you guys don't mind if I jump in. If all goes well, I'll be getting a 350 RWD next weekend (with 18's) and shortly after, I will need to get winter tires for the factory 18's. What are the staggered wheel widths? I would like to use a 225/40/18 tire on all 4 corners with the stock wheels. Is this possible? Any issues running a narrower tire in the back?

Originally Posted by magne

Never use the winter tires in summertime! This is a big no-no! All season tires will be a lot better in the summertime.

If you use a winter tire in the summer, you'll both ruin the tire and you'll have ruined the grip and handling of your car as long as the tires stay on the car. It is also pretty dangerous, as the winter tires have a very long braking distance on dry asphalt.
The mixture of rubber on winter tires gets destroyed if used in warm weather.

Go for the all season tires if you are using only one set.

If you want to save money, buy 17" after market rims and 225\45-17 allround, and don't use the tpms sensors in the winter.

If you don't mind spending the money I would do things correctly and fit 225\40-18 allround. The problems you'll have with 245\40 in the rear is that the ESP system might kick inn to fast. And there is no other logical reson for having wider rims and tires in the rear wintertime anyway. You are not going to benefit from the extra grip this gives when it is dry on the roads.

Originally Posted by magne

Only problem you'll get that might bother you is a sensitive esp system.