Can anyone check my method here?
My 997 C4S usually has 11" wide wheels in the rear, with a 50mm offset.
My snows are 10" wide with a 40mm offset.

To make the snows as far out as the summer wheels, I need spacers like this:

The 11" wheel is .5" wider over the centerline to start. .5" = 12.7mm.
Forgetting about offsets for a moment, that 11" wheel sticks out 12.7mm more over the center than the 10" wheel, before any offsets are added.

Then, that 11" wheel as +10mm more offset than the 10" wheel.

So to make the 10" wheel look like the 11" wheel, I need 22.7mm of offset, right?

Thanks for your help!

 

Nope. Your 10 inch wheels will be very close to the same as the 11s. This is because the 10mm lower offset on the 10s moves the wheel outward by 10mm, almost half an inch.
If you use a 2.7mm spacer, the wheels will "look" the same.

And you will have nearly an inch of additional clearance on the INSIDE.

 

Thanks for the reply. That's confusing me. I thought that a + offset meant that the wheel was sticking out by that much past the centerline. So a lower offset amount meant that the wheel was sticking out less.

(Based on this http://www.tirerack.com/wheels/tech/...jsp?techid=101)

So if two wheels are the same width (let's say 11") and one has a 40mm offset and one has a 50mm offset, won't the 50mm offset wheel be sticking out farther past the car?

 

Just the opposite. Wheels with higher offset are tucked further under the car. Less offset means the wheels stick further out. So an 11 inch wheel with 40mm of positive offset sticks out further than the same wheel with 50 mm of positive offset.
Adding a spacer always makes the wheel less positive. Add a 5mm spacer to your 10 inch wheels with +40mm offset and the offset becomes +35.

I know its confusing. I get mixed up sometimes and I'm in the business.

 

That's much for clarifying and explaining that! Very interesting.

 

I got 11/19/305 et 51mm wheels on my C4s has the same look as the 10/18/295 et40. I use the offset calculator, I thought I need a spacers end up it sticks out 1" from the fender.

 

A 3mm spacer would put you at exactly the same "poke" when running the winters. I wouldn't bother. Maybe put a 5mm if you're going to do anything.

 

Exactly my thoughts. I don't know that I could even notice a 5mm spacer. B/c of the negative camber in the back, it really looks like the rears are hiding in the wheel wells. Maybe a 10 mm would look best.