PDX958

Hello! I'm new to the forum, and also new to Porsche/Cayenne ownership.

I'm running the factory 295x45R21s, but recently bought a set of 20x9.5 OEM wheels.

I was simply going to go with the standard 275/45R20 tires, but after doing some research in the forums/manual and working through the tire size calculators, I'm still a little confused on appropriate tire sizes . Hoping someone can point me in the right direction. Here's where I get confused:

If I use the calculators to determine the outer diameter for the various stock/OEM Cayenne wheel options, I see that the ODs are different depending on the size.

295/35R21, OD=29.1"
275/45R20, OD=29.7"
265/50R19, OD=29.4"

So if I'm moving from 21s to 20s, I assume I should match the OD for the 20s rather than 21s?

But if I do that, is there reprogramming necessary to match the speedo, etc? Or does changing the wheel size in the TPMS setting take that into account? Or do I just live with the slight delta and not worry about it (it's only ~2%)?

Thanks in advance for helping out a noob!

deilenberger

Since the 20 and 19's are a larger diameter they might actually make the speedo more accurate. Porsche speedometers are known for inflating the speed shown. A larger outside diameter tire will help to correct that.

RS-America

Agree with Don, we do TSD Rally and really struggle with the Speed-o & Odo being off. We run 255-55-18 Grabber AT2s for three season and Nokian Hakkapeliitta's for winter snow & ice.
I made a conversation table using 5% plus 3mph that has gotten us real close. If my driving was more consistent, very close.
When we make the swap to BFG KO2s will have to recalculate the table.
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Crozzer

@RS-America hey Otis… I realize this is an old thread. I wondered if you ever figured out how to adjust the Speedo?

I did come across the yellow box and they claim it will work (I emailed them). But it is down in my list to explore it.

Any other workarounds, besides “just live with it?”

RS-America

.
Have not resolved the issue - our spread sheet seems to work well at TSD rally where accuracy is critical. Puts more work on the navigator which already is task saturated. Not perfect but better than relying on the speedometer.
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reneeweiss

I have a 2013 diesel with 18 inch wheels. The speedo reads 3 mph higher than actual speed. The dealer told me that,'in Germany the car company (Porsche) gets fined for speeding so Porsche sets the speedo this way'. I think it is baloney and I think the car was set to have 19 inch wheels but I am not sure. I haven't found a way to change the settings so i rely on the radar detector for speed or deduct 3 miles to get actual speed. The dealer is one of the best around in terms of service.

95ttht

It's true, modern Porsche speedos read high by 3-5% make you think you're going faster than you are (and to bail you out of tickets from speed cameras/radar)

In the dash menu settings, you are supposed to select your wheel size when changing wheels. One would assume they are programed for the diameter of stock sized tires, but I have never tried to confirm that it affects the speedo reading.

I'll try it as an experiment when I swap to winter tires in a month or so.

Crozzer

my understanding is this is primarily for damper tuning, though I’m not sure it is easy to feel the differences…. I’ll be interested to hear back if it does alter the Speedo.

so far, I think nobody on the forum has bothered, but that the yellow box may do it.