Hey Blu I found this post of yours on another thread: "Something to consider- 275's will perform better on a 10" rim. I was initially trying to stay with stock UCA's, which is why I went 18x9's. Knowing now that the stock UCA's didn't clear the rims at full lock and full droop, I would just have gone to tubular UCA's like I have now, and a 10" rim."
I'm guessing that a 10" rim and stock A arms would have contacted sooner than the 9"s do at roughly the same spots. Where are your tight spots in the front wheel well now? Is there room for a 10"rim/275mm tire combo in the front? What's your guess on where interference points would be? I'll be running stock spindles and 11.75 B body brakes for now.
Yes I believe the 10" rims probably will give you less room to work with than the 9's as far as the UCA's are concerned. I don't think stock UCA's and 10" wide rims are a compatible option, you just need too much backspacing to make it work. Like I said, my 18x9's didn't even work. They were close, so I think with a little less backspacing 18x9's might work with stock UCA's, but it will come down to rim design, exact backspacing, etc.
As far as the 275 tire, I think it will still work on a 10" rim, but your offset/backspacing will have to be
perfect. As far as "tight spots", pretty much everything is a tight spot on my car. ;) I'm still running my 3mm spacers with my 35mm offset 18x9's and Dr Diff 13" brakes. Right now I have a slight rub on my frame in the back at full lock. It's just enough to rub the paint off the frame, but the steering does hit the stops, it's not being stopped by the frame. I've got a set of 5mm spacers that I haven't installed yet to fix that problem. But, there's almost zero room to the fenders. I haven't hit the fenders yet, but the tires are right there at my current ride height. I haven't rolled them yet though, and I'm not running the most negative camber I could yet either. So, I'm holding off on the spacers until I'm ready to adjust my alignment or roll the fenders.
Tomswheels runs 285/35/18's on his Valiant, but he pushed the fenders out to do it. Even on my car I extended the lower front fender to bumper braces by close to 2" to push the corners of the front wheel openings out. The bottom front of the fender is visibly pushed out compared to stock. I also gave a little pull on the top of the wheel openings. If you sight down the bodyline on my fenders you can see they're slightly bowed out at the wheel openings. I may push on them a little more when I roll the fender lips.
So, it really depends on what you're willing to do. If you're ok with extending the fender to bumper braces, rolling the fender lips, and running close to -1* of camber I think the 275's on 18x10's will fit with the right backspace. You might not need to do all of those things, but you may need to do some of them. With the 11.75" rotors I would say you need to be at 25-30mm offset, or 6.5" to 6.75" backspacing on a 10" rim (I'm currently at 27mm offset, which is 6.1" backspace on a 9" rim). 6.5" would be the absolute minimum I would go as that will put the tires really close to the fenders, 6.75" will probably need a small spacer to adjust for frame rubs. It's a really tight window, and of course all of these cars are a little different. So, just because that's what works on my car doesn't guarantee it will work on yours.
Which is where "what you're willing to do" comes into play. If you're willing to play around with more negative camber, rolling the fender lips, pushing the fenders out with the braces, etc, then I think it will work. If you don't want to do
any of that stuff, I'm not sure I'd even suggest running 275's because of the body tolerances on these cars. They absolutely fit, but I definitely had to make some adjustments to get them to work. I think they're the limit without making irreversible changes to the bodywork. Where I'm at right now if you put the stock fender braces back on they return to their stock shape. Any further won't.
