I had been suspicious of the speedometer and odometer reading of my 2011 Cayenne for some time. So, I compared it to a Garmin GPS and with another car on the interstate. It turns out the Cayenne is high by 5%.
At 70 mph on the Cayenne, the speed on the Garmin and the other car was 66.5 mph. The mileage was high by the same amount as well. The 5% overage is consistent at lower speeds as well.
The tire size is 18” and the PCM is set for 18”. So, it isn’t a tire size issue. I called the dealer and they said Porsche sets the speed high on purpose and it’s a factory setting which cannot be changed by them.
Ok, I might accept a % or two, but not 5. Over the warrantee period of 40,000 miles, you’re basically giving Porsche back 2,000 miles. It’s a great deal for them.
So, you might want to check yours out. I’m disappointed with Porsche if they purposely set an inaccurate reading.
Regards

 

It's an EU issue. They have a regulation on the books there that penalizes manufacturers if their speedometers under-represent speed. The manufacturer's solution? Over-represent speed so you never pay the penalty.

I knew that was true on the speed but I didn't know that impacted miles too. Does seem more than a little deceptive if that allows PCNA to shave a few months off the warranty.

 

My CTT speedometer is also fast, at least 5, maybe 6%. Most car speedometers run fast, other than police cars which are calibrated within 1%. I think they are just trying to save you from a speeding ticket or two, but I agree, no good reason for the odometer to run high as well, other than to save Porsche money.

 

Reviving an old thread here. I just bought a 2016 958.2 Turbo S with sport chrono package. Factory spec tire sizes; 295 / 35 / 21, about 1/2 life left on them, properly inflated. During some extended highway driving, I continuously marveled at how it didn't seem that I was going as fast as my speedometer was telling me. I turned on my speedometer app on my phone, and sure enough, the speedo was reading about 6% fast - at exactly 100 MPH on the GPS app, the speedo read 106. I happen to have several different apps that read speed and they all read identical results. Is there any way to calibrate the speedometer to make it more accurate?
I'm considering swapping in 295/40/21 tires, which will get me really close to the desired revolutions per mile, but I'm not sure how that will affect my handling and wheel well clearance. Re-calibrating my speedometer would be much preferred if possible. BUT...is it possible? Anyone else try to fix this?