pdxjim

If you have an iphone, the MotionX GPS app will give you speed in mph or kph.

rbaron

Thanks. For some reason it didn't occur to me to look for iPhone speedometer apps and now I see (a) there quite a few of them, and (b) the new Cayenne's digital speed display is in fact at least 3-5 mph high at 60-70 mph on the expressway.

Who knew?

Apparently everyone but me.

skiahh

According to the GPS on my phone, our speedo shows about 7% high at all speeds. So it is linear and should be an easy change... if we could only get them to fix it!

So at 70 indicated, I'm going 65. At 30, 28 and so on. Easy to do the mental math, but when I get in the Boxster, its speedo is almost spot on and I catch myself automatically doing the 7% correction when it's not necessary, just because they look so similar!

It's annoying, but not fatally so. But I'd rather have it fixed.

morrisdl

Garmin GPS? Have you considered using your smartphone? There are lots of speed apps in all the app ecosystems.

I found my 2008 CTT speedo is 100% accurate. Much more accurate than any other measurements that I have seen derived though the drivetrain, so I suspected there is a signal from the radio nav unit to to improve speedometer accuracy (they always matched).

rbaron

See my post #32 in this thread and skiahh's reply in post #33.....I was on the highway for almost 4 hours yesterday and, having downloaded an app called Speed Box from the iPhone app store, I found the figure of 7% high on the digital display to be about right.

pdxjim

And you roll through the mileage warranty 7% sooner.

pbarch1

My 09 GTS Speedo is about 3 mph slow!! at 80mph, reads 77, according to my radar detector. which uses GPS

deilenberger

VERY unlikely.

The two are not necessarily related. The law in Europe prohibits any speedometer from reading slow. That's why many German vehicles read 5-7% fast. This is intentional by the manufacturer. People who have checked their odometers against highway markings, or GPS readouts - find them much more accurate. There can be a bit of discrepancy with a GPS unless you're comparing in Kansas which has no elevation changes.. the GPS measures linear distance, not actual travelled distance. Highway markers read actual travelled distance.

BTW - speedo in my '06 /S-**** was dead nutz on - compared to the GPS on a flat section of highway. The '11 CTT speedo is about 6% high.

wrinkledpants

You checked that your cluster is set to the right tire size and you have perfect air pressure?

You're going to get a 2-3% variance just from wearing a new set of tires down to being bald. Your tire pressure also changes your effective diameter by lowering the hub. There's another 1-2% if you're off by a few PSI. Heavy loads, cold weather, high speeds - these will all change the PSI from cold, and effectively change the rolling diameter. Was your tire in the shade or sun? Because there is a 5 PSI change in PSI if you aired up your tires in a warm garage, but then let them sit in the sun all day. That's another 1-2 % change in diameter.

So, you can effectively change the rolling diameter from 4 - 8% just with tread depth and PSI in your tires. In Colorado - you can be up in Breck where it's 30 degrees, check your cold tire pressure, attach a 7700 lb trailer, cruise into Denver where the temp is 75 to 80. Go to a meeting while the tires sit in the sun, and you've got an over-inflated tire by quite a bit. These are the situations that are perfectly within the operating envelope of the Cayenne. Vice versa. The Porsche dealer can put in the correct air pressure when it's 85 degrees out, and I can drive to Leadville towing a heavy trailer, let the tires sit overnight when it's upper 20's, and the weight of the trailer, air temp, etc - will have lowered the PSI and rolling diameter by quite a bit.

There is literally no way in the world any auto company can make a speedo be spot on unless they incorporate some type of PID control with GPS. Especially not a car that can see such a diverse vehicle weight and operating environments. Maybe if it was a Ferrari that was mostly driven in summer temps with a vehicle weight change of +/- a few hundred pounds - then you could get more precise.

user 83838290

I always wondered why someone would need a spot on speedo. +/- 5mph, who gives a damn?
Are you 85+ yr old, driving in FL and want to be 110% sure you're 10 below the limit? (which is still 98% above what such drivers can handle).

morrisdl

NC is writing tickets for +5mph or less. I got one for $25 fine (+$220 court fees).

driveline

Mine is right on the money. Original wheels and tires on a 9k mile 2014 958.

mcbit

Do you have non standard tyres as that is illegal on a european car?

pbarch1

They are standard GTS wheels and tire sizes, I do keep the tires inflated to the heavy load number, which probably contributes the the speedo discrepancy,

Petza914

Waze also has an option to display your speed in the bottom left corner, along with the speed limit on a lot of the roads so you can find out where you're going, where the police and shredded tires are, along with the police traps. You have to turn it on in the Settings menus.