IRRELEVANT INFORMATION , read down to 3rd post for resolution.

I need someone to confirm what wheel size will fit into a 285/35/18 the EXACT same way a STOCK 18x8 fits into the 245/40/18. It doesn't matter if they are even sizes that do not exist, because I am had some wheels custom made. I would like to choose my tire sizes first, rather than choosing wheels first, because it is harder to find a set of tires of different sizes that have the same overall diameter and sidewall height, as these are about the only two tires that have the same sidewall height. Been looking at pictures but it really doesn't help, sometimes a 285 will look stretched on a 10.5 wheel and other times it will look square, so I really cant tell the difference.

Remember these are going to custom wheels, so any size will be possible. Example 18x10.25" wheel. I just need someone whos knows what they are talking about. Any help is greatly appreciated as this is going to cost alot, and I dont want one tire to be stretched while the other fits square.

Please fill in the blank (any increment between 9.5"-10.5") :
Front - 18x8 - 245/40/18
Rear - 18x? - 285/35/18

 

nevermind I worked it out.

I simply took the tires sectional width and subtracted it from the rim width, then compared the tire amount leftover.

25.4mm for every inch, so a 245 width tire is 9.64" wide. A 285 is 11.22 wide.

245/40/18
9.64" - 8" = 1.64"

285/35/18
11.22" - 10.5" = .72" No
11.22" - 10" = 1.22" No
11.22" - 9.5" = 1.72" No
11.22" - 9.6" = 1.62" Perfect

Now, by comparing the leftover tire widths, I think that it is obvious that a 10.5" wide wheel will stretch the 285 tire too much compared to the 245 and 8". We will want just a tiny bit of stretch here since the 285/35/18 has a sidewall that is .14" taller than the 245/40/18. A 18x9.6" wide wheel would be perfect, but we still need that little bit of stretch to get it to an equal sidewall height. I do not want to work out an exact formula so we will just stretch it .14 of an inch, to take it off from overall diameter.

9.6" + .14" = 9.74"

So a 18x9.75" wide wheel is exactly what I am looking for.