19 captured repliesDiscussion pagination complete · Source accounts, not a confirmed diagnosis
THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCHSpeedometer reading incorrect... Fix?
Leading reported evidence
Run many many miles comparing the digital speedo
Performed · outcome unconfirmed
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0
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Read evidence and context
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Performed · outcome unconfirmed
I have run many many miles comparing the digital speedo to a high quality GPS's readings.
Leading evidence: 'Run many many miles comparing the digital speedo' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Replace diameter tires' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Suggestion · Original poster
Also getting near time for tires so if I HAVE to i could change diameter tires..
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Suggestion
If you are concerned about it matching perfectly then change the RD of the tire you use.
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Suggestion · Original poster
I am running 285 - 30 -18's on the rear (asume thats the one with the speed control) if not 225 - 40 -18's in the front
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Suggestion
Porsche may have done it intentially to help keep you from running afoul of the local polizei, or to run you out of warrenty a few miles early, but basically it is no big deal.
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Suggestion
Porsche may have done it intentially to help keep you from running afoul of the local polizei, or to run you out of warrenty a few miles early, but basically it is no big deal.
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Suggestion
(I don't think the speed display error is "reasonable", though.) This can be demonstrated two ways: One is to use the diagnostic mode on the EATC to display the actual speed in kph and observe the difference in the displayed speed.
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19 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
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Outcome without a verified action link
Love to get this fixed, though it keeps me below speed for the radar traps.
Hey all,
I have noticed the speedo is off on my 99 C2 by quite a bit (and not in the way that will help the re-sale)
I am running 285 - 30 -18's on the rear (asume thats the one with the speed control) if not 225 - 40 -18's in the front
Is there a way to fix this, and is it something I can do (some mechanical skills) or only dealer/indy
Also getting near time for tires so if I HAVE to i could change diameter tires.. but like the low profiles.
Love to get this fixed, though it keeps me below speed for the radar traps.
Ed
Many 911's are off. It's a common "problem." I don't think it affects resale in any way nor do I think it needs to be fixed. Mine has been off since I owned it.
-td
ALL speedos from factory are "optomistic." Yes, I understand your concern about the actual miles driven (Odo meter will register an equal percentage higher). But as strange at this may seem, the error is fairly consistent all the way up. I have run many many miles comparing the digital speedo to a high quality GPS's readings. Yes, you can fiddle with the tire diameter, but do you really want to screw up the handling in order to worry about 4 mph? Porsche may have done it intentially to help keep you from running afoul of the local polizei, or to run you out of warrenty a few miles early, but basically it is no big deal.
Beside, if you are driving your car for YOUR pleasure, rather than saving it for the next guy to enjoy, what real difference does a few percent really make?
If you are concerned about it matching perfectly then change the RD of the tire you use. Increase it if you want to bring the speedo back DOWN and increase it if you want to bring it UP. In your case it sounds like you might be going, say 57, when the speedo shows 60? If that's the case move up to the next size in RD. Just understand that you will affect your gearing by doing that.....make it taller (longer).
If it were about ending the warranty early, it would be a big deal because that is fraud. I used to think that was Porsche's reason. But the error is actually just programmed into the speed display, and the odometer is reasonably accurate. (I don't think the speed display error is "reasonable", though.) This can be demonstrated two ways: One is to use the diagnostic mode on the EATC to display the actual speed in kph and observe the difference in the displayed speed. The other is the display on my datalogger shows the error clearly in MPH. The datalogger reads the correct speed, so the speedometer is working correctly, and the speedometer display reads high, simply because it is programmed to.
I believe this 'error' is intentional on the part of Porsche.
Something to do with laws in Germany that require manufacturers not to understate speed (but my speedo said 55mph officer!) or face large fines. Therefore they play their hand cautiously and overstate the speed.
I've gotten use to having the speedo off, my car is 5 miles over and my wifes BMW 525 is 3 mph over. The odd thing is my 944 is spot on.
Rick
99 996C4
87 944S
I have check the odometer too and it reads the same (long) ni other words if I have 29 on the speedo and am going 25, then in 1 hour I have traveled only 25 miles but the odometer moved 29...
some simple math says then that my car with 104k only actually has about 90k this is a "little" error that can really add up. (hense my concern about resale on high milage cars.
Personally I dont think any 996 is worth less than mine with this many miles unless it is not drivable, rebuild or has some major mechanical/body/interior issues (as mine does not)
In this case I plan to drive the wheels off the thing and get as much out of her as I can.
Ed
is 5mph optimistic at 70mph (and below) and over 200 miles the odometer and a GPS device disagreed by just 0.2 miles. Over 50,000 miles this works out to 50 miles difference.
There is no law concerning odometer accuracy but auto makers are on their honor to maintain a +/- 4% or less error.
Honda got nailed on exceeding this and I think it was regarding leased vehicles and the miles the leasees racked up and in some cases were overcharged for.
While many Porsche owners have posted about speedo readings being optimistic no one to my knowledge has posted about any significant odometer errors.
Sincerely,
Macster.
Yeah, in the tenths of a mile. Most people don't set up experiments and compare the distance readings between odo and gps over long distances. Not as quick and easy as glancing down at the gps speed reading and seeing how off it is from the speedo.
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