I like the new 20" GT3 CL wheels (9x20, 55ET offset; 12x20, 47ET) and want to install on my 997.2 TT that presently has the 19" CL Spyder wheels
(8.5x19, 56ET offset; 11x19, 51ET offset).

My calculations show that the new fronts would create 5mm increase in offset while the new rears would create 8.4mm increase in offset.

Is that an acceptable number per Porsche specs?? THANKS

 

if by increase you mean that the ET values of 56 and 51 goes up to 61 and 59.4, then its not good. This would mean the wheels sit further inside than OEM. If it is the other way around, it will be flush up front but not very aggressive in the rears.

For a flush fit, the ideal offset would be ET 48-50 in the front and ET 35 in the rear.

Hope this helps.

 

By my calculations, the front would extend by 7mm and the rear by 17mm.
I'm not sure if these would fit or not.

Here is a great tool for calculating offsets.

http://www.1010tires.com/Tools/Wheel-Offset-Calculator#