So first let me say I really like my 2012 Cayenne S. Great road trip vehicle and I drive over 3K miles per month. However, the reliability has not been there - not a nightmare, not lemon status, but I've made far more trips to the dealer and not had use of the vehicle well beyond my comfort level. And a couple of the visits would have resulted in substantial cost had the thing been out of warranty.

So I'm sensitive to the warranty expiration, which will come up in another 10K miles or about three months. So I may sell the car earlier than I would have like to.

However, here is what this post is about. I've used two different GPS based speedometers on two 10 mile or so freeway stretches with the cruise control set. The speed on the car's speedometer is consistently 7.1% faster than the GPS speed, regardless of speed. I had a friend who has a Cayenne do the same and he got similar results.

I believe this means, among other things, that I will not get 7% of my legitimate warranty coverage.

Perhaps some of you could do this test. A good app is the Ulysses speedometer but there are several.

Give it a test and let us know if Porsche is up to something here . . .

 

Mine does the same and if remember correctly all cars do this some just more than others.

 

The speedo being off is normal and done intentionally for legal reasons (big fines if it under reports) in some countries. The average I've seen is in the 5% range, but my Cayenne is on the high side at 8% and I've seen as high as 13% before.

Your ODO is likely off a bit as well, but it shouldn't be quite as much. This variance will be due to your wheels/tires. Your reported speed and the ODO will be impacted as the tread is eaten away. Additionally it also depends on what tire/wheel package the factory tuned the ODO for and what you are running. For example if they tuned it for 19s with an all season tire and you are running 18s with low profile summer/sport tires or 22s with all seasons, then there would also be a discrepancy.

The tire/wheel example above is extreme. As long as you are using a tire/wheel package that is compatible with any of the factory options there should be little or no discrepancy when comparing equally worn tires.

All that said, unless you have have a really extreme tire/wheel variance I would not expect a significant variance.

For the speedo, manufacturers have "acceptable" limits and as long as it's in that range the dealer won't mess with it. These days I'm not even sure what they can do.

I don't know what variance is acceptable/legal for an odometer.

Another thing to consider is that consumer grade GPS units aren't 100% accurate either for a few reasons (available satellite resolution, how GPS itself works, etc..). I've run multiple "identical" units right next to each other and gotten different results from each.

 

I don't think GPS variance applies; given a 10 mile test. Here is the question - do we ever see them where speed is overstated? If not then we really don't have 60K warranties do we? If I had a big repair at 64K miles I think I'd have pursue a remedy on that. By the way my tests were with virtually new tires, same brand and size as factory and at factory recommended inflation.

 

When using a GPS to get your "real" speed, of course its variance applies. The number of satellites it is talking to greatly impacts its accuracy. The fewer satellites it talks to, the less resolution it has.

I have no idea if people have tried fighting warranty expirations due to the variance, but I would be surprised if someone has won due to the difficulty in proving it has been a problem since new and the variance has been consistent the whole time. If you are going to try to fight it, you need to get it into the dealer ASAP to get your accuracy concerns into your records. Unlike them trying void your warranty where they are required to prove you are at fault, in this case you have to not only prove there is a problem but also proven that it amounts to (in your example) to a 4k difference. Should something happen, your best bet it to talk with your SA. The more non-warranty work you've had them do, the more they tend to try to work with you.

Probably your best test will be a reliable friend with a stop watch and get your times going past some measured mile markings on the highway.

As far as under reporting, yes it does happen rarely. The dealers/manufacturers take that extremely seriously given the penalties they face in many places (e.g. they want to make damn sure its not a systemic problem).

In the end though I would say that if you are that worried about the warranty, then you need to either get an extended warranty or sell it and move on to your next vehicle (Cayenne or not).

I'm all for fighting for the principle of the thing when you are getting screwed out of your rightful due, but even I have my limits

 

gnat, I don't want to be argumentative but this GPS thing is something I know quite a lot about.

When you develop a speed measuring algorithm using GPS points the error is not cumulative. Its not like dead reckoning in which error is accumulated. A 100 foot error in any measurement point would be quite unusual so lets use that as an extreme. The algorithm would be written so it takes many measurements in the mile. The maximum error however remains 100 feet. So in a mile the maximum error would be 100 feet; or 2%. If I go 10 miles the maximum error remains 100 feet or .2%. But in practice the error will average fewer than 50 feet, so the accuracy will almost always be more accurate than the dead reckoning method of a car's speedo (wheel revolutions against time). Surprisingly, the timing element variable in the calculation is responsible for more of any calculation error than is the triangulated coordinate. Three satellites can nail the location reliably to within a few feet.

In a law suit the first thing an attorney would do is subpoena manufacturer records to determine company was aware and if the inaccuracy is intentional.

 

Based on your usage you appear to be losing 5 weeks of warranty. Since the repair visits are bothersome I would start to get rid of it now. Get your next car's speedometer calibration checked at shop like Prorstar Calibration in Palm Springs.

Just wondering...if you are using navigation isn't it GPS and when using your app wouldn't it be tower triangulation?

 

I could be all wet on this, but I seem to recall reading somewhere that the speed shown on the speedo is not the speed that the car "knows" it really is going. Hence, mileage reporting is accurate, speedo is intentionally shown faster than normal.

I am pretty sure this is the case in my BMW, where when the digital in-dash speedo is uncoded, it reads "true" speed, which is slower than indicated on the dial.

In any event, if prompted, I bet this is the answer the dealer gives....true or not.

 

What size wheels are you and your friend using? Maybe the problem is more associated to the wheel tyre size?

 

I'm on 18-inchers, and have a consistent +3 mph speedometer across the board. GPS says I'm doing 32 when the speedo says 35, 52 when it says 55, 67 when it says 70, et cetera. That's a -4.76% error at a GPS-verified 60 mph. I suspect your 7.1% may be related to wheel size. FWIW, I believe this error likely explains the difference between indicated and calculated fuel economy (mpg) as well

//greg//

 

^^^ This is also in line with my testing, i.e. mph difference of observed versus computed by speedometer. I have a Stalker radar gun (k-band) and have tested my 2013 CD at various speeds and compared to the Stalker reading. My CD is consistently 3 mph above the Stalker (for comparision my 997.2 is 2 mph above the Stalker) at Cayenne speeds ranging from 30 mph to 75 mph. I also did a distance test for two separate mileage blocks of 50 and 100 miles on I-5 from SF to LA using an iPhone GPS app against the odometer (not very scientific, it was more a curiosity test). I found the error again in the 4% range. As these two tests seem to indicate a consistent measurement error in both speed and distance it appears my CD speedometer and odometer are reasonably linked. And I agree this translates into inflated MPG readings from the CD.

I also did a short test by changing the tire size in the onboard computer -OBC- from 18" (my actual tire size) to 19" and 21" tires the OBC "thinks" I have just installed. My speedometer error increased by about an additional 1 mph with the 21" setting.

All in all, I can live with the error and knowing the magnitude I can adjust as needed/wanted.

 

just stumbled on this thread, after noticing my speedo is about +7% high compared to both my separate TomTom GPS as well as iPhone app (both pretty much sync with each other).

I have 19-inchers on with OE tire size and the system is set for that.

I do understand that Porsche and others may be intentionally miscalibrating speedos for EU markets, but I am in Canada, and would consequently expect the same accuracy that I can get in a 25yr old Japanese or domestic vehicle.

I have an appointment with the dealer on Wednesday and have mentioned this issue. It'll be interesting to see if they will, or can do anything about it.

 

I tested this on our recent road trip and saw 2 MPH difference under 50 MPH and 3 at speeds up to 90 indicated.

Honestly the slow speed one was only done for a short period so it could be a consistent 3 MPH.

20" factory wheels.

 

So, just back from the dealer and they told me nothing was wrong...basically live with it.
FWIW, mine is a 09S, and their loaner was a 2013 V6 which was also showing about 6% faster than my GPS and iPhone.
I'm really feeling that the miscalibration is really huge and it kinda bothers me. Next step is to check the odometer's accuracy...

 

Tell the dealer you'll live with the speedo area if they will live with extending your warranty by 4200 miles (7%)