I recently purchased some OEM 21's to replace the stock 19's on my CTT. I haven't measured them yet, but by looking at them, the 21's are a bit taller than the 19's. Will I have to get the speedo recalibrated for the taller tire? Or is there a way for me to do that myself?

I did a search for this using various key words: wheels, speedo, speedometer, recalibration, etc, but was unable to find a proper thread.

 

I too would like to know, I have 22" on mine and speed I believe is accurate but now you have me wondering....

 

You might have a 3-5mph deviation with the larger tires. I've used a drift box to see what my speedo said.. .(22inch TA's) and it was off by about +-3mph.... not enough to go through the trouble of recalibration.

 

I was using "the Google" to try to find some answers and there was mention of something in the computer menu about changing tires and it giving you options of tire sizes. I'm at work right now and not able to check the car to see about this. Maybe someone has looked through the menus and found this setting? I'm going to check when I get home and will post the results of my search.

 

Ok, so I got home and installed the new tires and wheels. Then I got in and turned on the key and went through the menu. I went to settings and then tyre pressure and then settings, IIRC, and I was able to change the tire type from all-weather to summer and then I was allowed to select the rim size. As soon as I checked the box for 21", it made a beep and then said "No pressure monitor" and "Learning". I had read that you sometimes have to drive them for a while, up to 10 miles, to get the TPMS to register. So I backed out of the garage to go for a drive. It registered and popped up the tire pressures before I got out of my driveway!!! I took it for a quick spin and everything seemed fine. I don't know if the speedo is off, but it would seem that since I was able to select the wheel size, it should accommodate for that. Obviously, Porsche was thinking that people would be changing tires and wheels for various conditions since this was part of the computer menu.

 

A couple of things.

First, unlike my location, I believe you guys have access to some sort of GPS that can measure your actual speed and you can use that as a reference to see if your speedo reading is correct or not. That is a technique used by members of BMW forums. Some of them actually reported that their speedo reading got more accurate by going to their larger sized tires as BMW cars are known to have a faster-than-actual-speed speedometer reading. I don't know how accurate Porsche speedos are compared to actual but, yeah, you can try to use GPS to get your actual speed and you can go from there.

Second, when one switch to larger diameter rims, I believe that OD should be maintained by getting the correct sized tires to maintain factory accuracy of the speedo, odo, wheel sensors (ABS, traction control, etc). One can use the tire size calculator at 1010tires.com to find out what size of tires for the larger diameter rims would yield the closest possible OD to stock. I believe that the wheel & tire settings in the menu would assume that correct ODs are used despite different rim sizes & tire types.