X25

The main advantage of wide-body C7s is the ability to run wider tires and increased track. There's not much to do for the increased track besides adding wide-body to your car, but looks like you can run the same width tires!

So what do you need for a very wide tire to work without running into any issues? Only two things, but it's easier said than done : )

• Body control: Wider tires increase traction, and make the situation even worse. We need a lot more body control, and there are a few major ways to do it: springs, matching shocks, and sway bars. I believe way bars alone can do wonders on body control.
• Clearance: Custom wheels with perfect offset goes a long way. The fenders can also be massaged for more clearance. Aftermarket suspensions that limit droop make it possible to use perfect offset, since they reduce the potential of wheel interfering with rear upper control arms. Finally, camber helps.

My recipe:

• Z51 w/o Magna-Ride (allows me to easily modify the suspension)
• Custom wheels 18x10.5 ET52 front, 18x11 ET79 rear (BC Forged or similar; build-link)
• Penske coilovers w/ 750 lbs/in front, 650 lbs/in rear springs (build-link)
• Not too low ride height (mine is at 0.6" drop from OEM).
• LG G7 sway bars (build-link)
• Track alignment.
• Hoosier R7 315/30/18, 345/35/18 or similar.
• Optional
  • Granatelli rear adjustable toe links (to hit rear camber goals; link)
  • Fender lip removal (insurance; link)
  • Adding wheel spats for front (rock protection; link)
  • Knurled bead option for custom wheels (prevents tire slip; pic)

Remarks

• The project car, magna ride cars: If you have magna ride, you're very limited on suspension mods, since it is apparently very hard to disable them on C7, unlike C6. There are simulators available, but I've heard that they fail in time. I'd love to learn otherwise.
• Coilovers: Body control is of utmost importance if you'd like to run wide tires, and this includes any C7. Even wide-body cars run into rubbing issues at times, since OEM shocks are not valved as aggressively digressive as most track-focused coilover shocks. Coilovers allow us to choose from thousands of coilover springs rates available, and get a shock tuned for it. Without coilovers, the body control would be left to sway bars only, and the car might run into rubbing issues in tight corners, if not at the street. Besides all this, I've learned hard way that it is critical to match the suspension to tires. If you run very high traction tires without an adequate suspension that comes with matching springs, it will be far from optimal.
• Sway bars: You may or may not need this, depending on how stiff your springs are.
• Why 345s, not 335s: This has been a learning experience; I actually have sourced both sizes for testing. Stingray fenders do not let you install any 18" wheel wider than 11" without running into rubbing issues. When you look up sizing recommendation for 335/30/18 R7s, you'll see the range is 12"-13". Judging from that, one would think 345s would require even wider wheels, but just look it up, you will see the recommended range is 11"-13". Wait, what? The reason is that the much longer profile of 345/35/18 allows the tire to conform much easier to the 11" wheel, while the short profile of 335/30/18 forces the tire to substantially pinch.
  
  This is 345/35/18 A6 on 18x11 wheels.
  
  
  This pic shows how the larger profile conforms to the narrower wheel. Observe the middle line.

From my build thread:

Jan_N

Any update on the big tires on the track.. Any rubbing?

X25

From my build thread:
Well it worked!

• Outside: I did not see any rubbing outside, and trimming inside lip of the fender seem to have helped.
• Inside:
  • I did see a bit of rubbing of fender liner inside where it protrudes, though.
  • Upon further anaylsis, I realized I could have avoided it by going with 18"x11" ET76 wheels instead of ET79. Probably not so coincidentally, ET76 is also CCW's recommendation at this size (and I trust their advice).
  • Long story short, I'll be using my current set with 3mm spacers in the back with these tires, next time. I was already going to order a second set of wheels, and I speced them ET76 to avoid having to use spacers, too.

In short, I recommend 18x10.5 ET51 Fronts, and 18x11 ET76 as the optimal set for these tire sizes. Keep in mind, you'll also need either sway bars, or stiffer springs to mute the excess suspension move these high traction tires create, or else it might rub.

Happy tracking!

Riderover50

X25

Yeah, body motion is critical. Do you have aftermarket sway bars by any chance? Stiffer sways do a lot in limiting the body lean, including during bumps.

It's good to learn that MRC update softens it more, btw.