Charles
Congrats! With the Source add ons you should be good. We installed much larger tires and will report our findings once we've driven enough miles. RR4R chassis (you have one) are notorious for the passenger rear being much heavier than driver rear. This effects tire pressure and the only way to find out how much you are heavier is a four corner weighing that usually cannot be done at a public scale. If you are coming up on needing new tires, and should be if they are oem tires, you might learn from our experience. Also, before you pony up for new tires, get a quality alignment done if the original owner did not have one after replacing the trailing arms. Tires cost too much to ruin prematurely. If you will be anywhere near Orlando, Josams does the best quality alignment and they also perform a four corner weighing. The problem with our oem tires was with the extra weight on the passenger rear (2700lbs) meant all four drives had to be at or near max pressure, with the passenger duals also near max weight rating, making each bump a hard one. Bridgestone Ecopia R268 295/75R22.5 is 2 sizes up and a huge tire for our Cayman--but it fits fine and does not rub, and holds more weight at less psi and therefore smooths out the bumps. Speedo changed and now awaiting news from Allison to see if we need to program the tranny--do not think we do. Good luck, feel free to contact me offline
flynnwalter@yahoo.com so we don't have to repeat my previous postings here. Hope this helps you.
Frank W. 09 Cayman 38SBD
Lake City, Florida