pearstar

Hi everyone,

I just noticed something about the mileage reading last weekend. I was on a trip that was 291 miles, but Cayenne gave me 301. First I did not pay attention to it but when I got back on Sunday, Cayenne gave me 296 on a 287 trip. Then I realized there might be something wrong with the mileage reader. It just seems weird that this could happen to a Porsche. I did this trip a lot, my tire pressure was good, I am very familiar with the road and the other vehicles I used to trip do not have this problem. (they have a <1 difference which is acceptable) So is this a common issue for Cayenne?

It's a 2011 V6 BTW.

RESP

Not sure what the regulations are in US of A, but in Canada, they can produce vehicles with a variance up to 7% from reality.
Your numbers seem to be within 4% of reality.

Of all the cars I have owned, very few read on the speedometer/odometer the real speed or distance travelled.

I typically use an accurate portable GPS unit to find out what I am "really" doing for speed, so I can adjust my speed accordingly.

Funny thing, our Lexus is within 99% accuracy

mcbit

I'd consider you to be somewhat overcritical if you consider <1% to be "accepable". New to worn out tread wear on a tyre with a diameter 28" will show a difference of 1.7% during their life. The difference between a new 295/35R21 (29.2") and a worn 255/55R18 (28"-0.5") is 6.2%, both of these tyres are fitments recommended by Porsche on the Cayenne. Unless the ODO is linked to GPS for calibration the ODO will also display these differences.

Both of mine when checked by GPS are doing about 140 kph at an indicated 145 kph (3.6%), and with the turbo at 227 kph indicates 240 kph (5.7%) which I find to be more than acceptable when compared to a few of my friends and colleagues' cars which are typically off by 5-10% at 140 kph. However on my GTS with a factory fitted Porsche Nav system which does not display the speed but displays the distance from home to work as 131 km, my ODO also shows 131 km on arrival. It seems that the ODO is recalibrated against the Nav if the difference strays out side of prescribed limits.

xwxman

My 2012 Cayenne needs to have the cruise set on 78 to travel between mileposts
in 48 seconds (true 75 mph). This is a 4% error. So it seems this would be reflected
in the odometer reading 4% long at 75 also. BTW it is fitted with 18' wheels and ContiSport
summer tires.

Bill P.

For the record, my CS speedo also runs close to 3mph over actual speed.

mcbit

Not necessarily true, the displays are digital and if referenced to the Nav could be output independently. In that way you can have accurate mileage tracking and still have the speed enforcement safety of having an overreading speedometer.

xwxman

Not necessarily true, the displays are digital and if referenced to the Nav could be output independently. In that way you can have accurate mileage tracking and still have the speed enforcement safety of having an overreading speedometer.
__________________

This may be the case, does anyone know for sure? Wish the Nav would
show the GPS speed in the corner the way my old TomTom does

deilenberger

How was the expected distance for the original trip gotten? Many map programs don't really account for changes in elevation, which can add a significant amount of distance to a trip... (ie - road going up down hills may only travel 10 miles as a crow flies, but could be 11-12 miles in actual pavement covered.)

My '06 is dead nutz on. It agrees with it's internal GPS, it agrees with my Garmin GPS, and I found some long distance MEASURED milage test zones out in the west (10 miles was fairly common) and it was dead nutz accurate with those. I'm running 275-40/19's (Mich Latitudes) about 65% worn. The reason I emphasised "MEASURED" - the typical mile marker isn't. It's simply laid out using some vehicles odometer. Supposed to be accurate, but the measured mile ones are measured - and there just for the purpose of checking your odo and speedo. Dunno of any of these in the NJ metro area..

mcbit

On the GTS with factory Nav the speedometer is not 100% accurate but the ODO appears to be pretty close. This an extract from the manual:

When put into operation for the first time, a distance of approx. 3miles (5km) must be driven in order for the navigation system to complete the process of fine calibration. The same applies when the tires are changed (e.g. summer/winter tires) or new tires fitted. Full location accuracy is not yet achieved during the fine-calibration process.

Unless ODO and Nav are linked there would not normally be any concern on changing tyres.

Audis also do not show speed on the factory GPS, which Ipresume is deliberate on the part of the manfacturer to ensure that the customer is not confused.

Slow Guy

From my experience BMW's (esp newer BMW's) are the worst for speedometer error. The factory tolerances are ridiculous, something like 10%. They claim it's because Euro law penalizes them pretty harshly if the speedo reads under what the vehicle is actually traveling so they fudge it on the conservative side. Thing is, no Porsche, MB or Audi I have driven has been anywhere near as off as the wife's last 2 BMW's. I hate it, major league annoyance for me.

deilenberger

I'll agree on BMWs error margins. It's 10% plus 5MPH - so it's speed dependent. It carries through on the bikes also.. That's why you hear many BMW owners saying "Boy - never felt like I was doing 80.." because they were actually doing 68MPH (actual error on SWMBO's 5-touring..) and being passed by everyone on the road.

pearstar

I got the expected distance from google map. And yes I already thought about the terrain, and I did the math, if you have an extra 10 miles in a 300 miles trip, that means the road has an average 15 degree up/down angle, which is not very likely in mid Ohio.

My car is 2 months old and so are the tires (18' all season, 3000 miles), and I don't have a NAV to compare the speedo. (wow...so I was not really doing 85??)

I'm not trying to be picky but some people may worry about their warranty or resell values. After all, if your car's odometer reads 5% long you "lose" about 2500 miles at 50000 miles. Even 4% will cost you 2000 miles. Bad news for renters.

Actually I heard from somewhere that the trip computer works different from the actual odometer on the dashboard. Will give it a try this weekend.

deilenberger

Been to mid-Ohio (just a week ago) and any incline in excess of 0.015% would be considered a mountain there. I do find Google maps to be off somewhat compared to GPS recorded mileage (which also has a problem with elevation changes..) usually giving me a shorter distance then the actual. I'm sure someplace on Google there is a long writeup explaining how rounding errors or something like that will cause this.

I'd suggest borrowing a GPS - then plot a Google route, and record it as a trip on the GPS (and on your GPS in the Cayenne if so equipped..) See what the error is.. and perhaps where it is.

Shawn Stanford

Just to ask: Are your tires inflated correctly?

mcbit

On the 05 CTT I have completed 15,600 km since last service according to the ODO. My GPS log reset at last service is currently showing 15,680 km or a 0.51% difference (17' /mile) despite there being a difference of 4.25% between Speedometer speed and GPS speed.