A couple of wheel/tire questions

[grannyknot]

The car, an 08/Z4MC has on it right now style 359, 19x9/ET31 with 245/35 Pilot sport A/S3 on the fronts and 19x10/ET25 with 275/35 on the rear.
There is no rubbing and it's a good looking set up.
I will be spending a lot of time on the Hwy and the ride is a bit harsh on a rough road so was thinking about going with 18" wheels to gain that extra 1/2" of side wall. Do you think there will be a noticeable difference in the ride with extra 1/2" of rubber?

Also, I noticed the dia. of the front tires are 654mm and the dia. of the rears is 675mm, isn't that difference going to show up in some detrimental way?
I can't say I have noticed any problem with the difference if dia. but it just seemed to me the they should be the same ... or I could just be anal.
Thanks

 

[Mr Tidy]

When I bought my MC it had CSL Replicas fitted, so 8.5Jx19 ET44 fronts with 235/35 MPSS tyres and 9.5Jx19 ET27 rears with 265/30 MPSS tyres.

I think the idea behind having a lower profile on the wider rears is to keep the diameter similar to the fronts.

While I loved the style of the CSL Reps there were just so many Z4s running them I decided to go back to original 224s and found a set less than a mile from home! So after getting them refurbed I got new tyres fitted before I put them on the car.

So now I have 8Jx18 ET42 fronts with 225/45 tyres and 9Jx18 ET30 rears with 255/45 tyres. I went with Kumho Ecstas after reading some positive feedback on here, but went with 45 profile on the rear instead of the standard 40 profile for a bit more ride quality and they do fill the wheelarches pretty well!

Anyway to answer your question the ride is definitely better on the 18s, but it isn't like night and day better. So I do think you'd notice a difference on 18s, but not massively.

My car is on H & R coil-overs set as high as they will go, so 0.9" front and 0.7" rear lower than standard and sits like this on the 224s.