A wider arch adds space at the outside of the wheel well and nothing at the inside. The tyre now sits further out, but the exhaust pipe, the liner and the filler neck stay where they were. Clearance problems usually appear on the inner side, not the outer.
So a body kit does not automatically create room for a larger exhaust or a wider tyre. It changes where the outer limit is, which changes what the tyre can touch when the suspension compresses and the wheel is turned.
Plastic liners are the first thing to move when an arch is widened, and the first thing to rub. A liner pushed outward sits closer to the tyre at full lock, and a liner that has been trimmed leaves the back of the arch exposed to spray and stone damage.
Check the liner at full lock in both directions with the suspension loaded. A mark on the liner is normal. A liner worn through is a sign the tyre is running where the arch does not agree with the suspension.
A rear skirt or a diffuser changes where the exhaust tip can exit. The cut has to be made after the tip is positioned, not before, because the height of the tip depends on the hanger and the hanger depends on the shape of the new bumper.
Cut small and open up. A hole slightly too small can be filed to sit around a titanium exhaust tip and will look intentional. A hole that is too large needs a trim ring or a new panel.
Lowering the car changes the geometry, and a wider arch usually comes with a wider wheel. Together they move the arc of the tyre closer to everything behind the liner, including the exhaust where it runs past the wheel well on that side.
Measure that gap with the car on the ground and the suspension compressed on one side, not with both wheels hanging. A static gap on a lift tells you very little about what happens over a crest with the car loaded and the wheel turned.
Body kits and arch extensions are usually fixed into panels that were never designed to carry them. Self-tapping screws into thin sheet loosen, and rivets into a single skin pull through. The load is small, but it is vibrated constantly for the life of the car.
Use the existing threaded points where possible, add a backing plate where there are none, and keep fixings away from the edge of a panel. A fixing close to an edge tears out rather than loosening, and it takes paint with it.
Fit everything unpainted first, drive it, then take it apart for finishing. Movement under load, contact with the liner and the final tip position all reveal themselves in use, and all of them are cheaper to change before the panels are painted.
On the exhaust side, check the pipe where it passes the wheel well, with the arch fitted and the suspension loaded. A run that clears a standard arch can touch once the liner has been trimmed and a wider tyre is using the same space.
