Quote Originally Posted by rginah1974 View Post
I 100% trust u on it, I just wanted to understand where my mistake in the calculation is.

With regards to standard 18" sizes. I took the measurements from FFR homepage, when you configure the kit. Here I copy/pasted it from the homepage:
16693 - Vintage Halibrand Replica Wheels, 18” X 9” Front and 18” X 11” Rear & Mickey Thompson Tires, 245/40R18 Front and 295/35R18 Rear+$3,699.00

I guess there is a mistake there, so I will use the 315, instead, thanks for noticing it to me.
I will go with:
255/35-18; 25.0" diameter, 10.2" section width and 9.3" tread width
315/30-18; 25.5" diameter, 12.6" section width and 11.4" tread width.

And yes, i made a typo on the width for the 15" tire. Thanks for pointing it out.
That is funny. Just before typing my post above I went to the FFR website and looked in the roadster parts section for wheels. There they list 315s for both 17 and 18. I haven't messed w/ tires on a MkIV but on my old MkII the diagonal tube that runs behind your shoulder out to the door latch area was a problem for larger diameter tires. So if thinking about larger OD than the standard 315s, I'd ask if others have tried it. Also note that for a given tire size, tires of different brands can vary more than you might think. Way back when people were trying to fit 295-15s on MkIs and MkIIs it was common to try to choose the narrowest 295. Tread width could vary by a full inch. We were usually worried about thread and section width but I don't remember much discussion about OD.