I have a 991 RS and would like to put Hoosiers on the car that are now available in 20 inches.

I have read a lot of the posts about wheels being made and tires for 19 inch rims and the problems with the calipers.

Now that the 20 inch tires are available I have a few questions

1) Will the 991 Gt3 rims fit on the GT3 rs. I realize we would lose a half inch of width both front and rear but this should not be a problem with the different size Hoosiers available. Most wheel makers do not make a 20 inch 9.5 and 12.5 anyway hre/forgeline/bbs.

2) Could I use the fronts from my rs 20x9.5 or would that make them too wide to turn without dragging the fenders. This way I would not have to mess with offsets and differences in the hub. And then only buy new 20 inch Gt3 rear rims and run 20x12.

Trackdaytire.com has a full complement of the 20 inch sizes.

I have read about the different rolling ratios and we should be able to work this out with tire sizes. I have also been told that a half inch from a sport cup and this loss of 1/2 inch on the Hoosier would not be an issue.


thoughts...

Looks like the 20”sizes listed on that site will work for GT3. You are losing a half inch radius from both front and back due to lower profiles. But in the RS you also lose half inch in wheel diameter dropping to 20” from 21”. So the radius on the rear loses 1”, half from the tire profile and half from the wheel size. That seems huge, and the effect on rake might affect the front to back aero. Maybe you can adjust the rake to compensate and restore the aero balance. I am not familiar enough with the RS to know if it has that much adjustment. Probably okay, but I would check with a Porsche knowable shop.

Unfortunately there aren’t any physical dimensional specs on the 20” sizes so we are really guessing at the fit based on nominal sizes.

Thx for the replies

I think the rake can be fixed/adjusted without to big a problem but the big question I one that you asked.

Does the Hoosier tire with a half inch less rubber in the rear provide more traction than sport cups with one half inch wider width?

The greatest concern for me is the rolling diameter and revs per mile that would mess with the traction control. It's an expensive experiment if you only have one size to work with and it sets off the tc because of these issues. I have been told the computer is a learning system and it will learn the different sizes and work with them as needed automatically.

I was really wanting to swap the rear wheels from the non rs car and save a few bucks.

Deacon,
Looks like a direct replacement for the GT3 sizes! Nice! Finally, agreed!

For the RS, the rear tread width is over an inch narrower and overall diameter is 1.6” smaller, a significant contact patch reduction. The tires are stickier but not sure that makes up for the smaller contact patch. The real concern is even though the front is also narrower, it is only 0.6” narrower so the front will not lose as much patch as the rear, shifting adhesion balance to the front. Overall adhesion could stay the same or still go up and the balance front to back might be adjusted for also with bar adjustments. IDK, need to test. Need an RS!