10-19-2007, 07:42 PM
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#1
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Registered User
Join Date: Apr 2007
Location: Ontario
Posts: 32
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Speedometer accuracy
During my drive today, I was using a GPS. The speedometer was consistently reading 8-10% higher than the GPS.
Is there a way to have the speedometer adjusted/calibrated to provide accurate readings?
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10-19-2007, 07:54 PM
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#2
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Porsche "Purist"
Join Date: Aug 2003
Location: Wisconsin
Posts: 2,123
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These cars usually read a constant 3 to 4 mph faster than actual. The odometers are usually correct.
Others have reported the dealer can reduce the error somewhat, most of us just drive 10 over.
__________________
1998 Boxster with 7.8 DME, 2005 3.6 liter/325 hp, Variocam Plus, 996 Instrument panel
2001 Boxster original owner. I installed used motor at 89k.
1987 924S. 2002 996TT. PST-2
Owned and repaired Porsches since 1974. Porsche: It's not driving, it's therapy.
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10-19-2007, 08:25 PM
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#3
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There Is No Substitute.
Join Date: May 2007
Location: West Coast
Posts: 3,253
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My GPS is normally reading 2-3 MPH below my speedometer. It depends, on the car, most of the speedo's are off a little, but it shouldn't affect the odometer. If you ever upgrade rims sized or something of that sort, that could cause it be off. The dealer might be able to help you calibrate it. I just go with the flow, and avoid being the fastest on the road (unless no one is around) and the exact speed doesn't really matter.
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1999 Ocean Blue Metallic Boxster - blueboxster.com
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10-19-2007, 08:32 PM
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#4
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Guest
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i hear the dealer can change it because its electronically controlled. In my car I can see the real speed (in KM/hr) using the climate controll special function.
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10-20-2007, 02:56 AM
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#5
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Guest
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My GPS says it is off (fast) by 2 MPH in most speed ranges. Same thing for my Ford Explorer.
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10-20-2007, 03:21 AM
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#6
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Registered User
Join Date: Sep 2006
Location: Naperville, IL
Posts: 793
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According to my GPS and the radar/speed signs by the side of the freeway my car's spedometer is about 4 mph over my actual speed.
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10-20-2007, 04:58 AM
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#7
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Registered User
Join Date: Apr 2006
Location: Dallas, Tx
Posts: 730
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Quote:
Originally Posted by Paul
most of us just drive 10 over.
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Very true but for most of us that doesn't have anything to do with speedometer accuracy.
__________________
2003 Boxster - Sold but not forgotten
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10-20-2007, 04:59 AM
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#8
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Registered User
Join Date: May 2007
Location: Manassas, VA
Posts: 203
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can somebody explain to me how our speedometers can read 2-4 high but the odometers will still be fine? I don't uderstand!
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10-20-2007, 05:16 AM
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#9
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Registered User
Join Date: May 2007
Location: Suwanee GA
Posts: 87
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I would like to also know that. If that is the case there would have to be two separate circuits, one for the odometer and one for the speedometer. One way to test this would be to drive a steady 50MPH for one mile. If the odometer flips at the end of that time then it looks to me like the odometer is also off.
Tom
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10-22-2007, 03:37 AM
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#10
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Registered User
Join Date: Jan 2007
Location: Australia
Posts: 4
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admlmm
I can't answer your question about the difference between speedo and odometer but I can share my experience. I have fairly good Garmin GPS and it reads 90kph at 97kph on the speedo, and 110kph at 118kph on the speedo. This means the speedo on my 2002 S manual reads 7 to 8% too high. This is crazy given I have 265/40's on the rear instead of what the manual states should be 265/35's. This means I have bigger tyres that matematically should mean the speedo under-indicate by about 4% yet it seems to over-indicate by 7 to 8%. Must be how Porsche set good fuel economy figures and longevity data.
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10-23-2007, 04:44 AM
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#11
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Registered User
Join Date: Jun 2007
Location: Indianapolis
Posts: 8
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Mine's a 2001 S, and its speedometer is off by a constant 3-4 mph at all speeds above about 20mph.
The odometer is accurate. Odometers have to be accurate as a too-optimistic odometer will run out the warranty more quickly. One of the Japanese automakers (Toyota, I think) lost a class-action suit over this odometer issue.
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10-23-2007, 05:36 AM
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#12
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Guest
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I used my Garmin GPS this past weekend to check the accuracy of the odometer too. It's off by a little.
For a 300 mile trip on the odometer, the GPS read about 297.5 miles.
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10-23-2007, 06:35 AM
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#13
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Registered User
Join Date: Jan 2007
Location: Depends on the day of the week....
Posts: 1,400
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My speedo is 4MPH higher than GPS indicated speed consistently. I've also found this to roughly the same error in the odometer, when comparing trip figures from the cluster to trip legs from my GPS.
Patrick
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05-06-2009, 08:10 AM
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#14
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Registered User
Join Date: Oct 2008
Location: Ohio
Posts: 868
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True Speed
I thought it was a joke when I read that your HVAC controls could be used as a diagnostic tool, and to tell you your true speed. But, I tried it on my '98 and was truly amazed to find it was true. I checked the speed at 60, 70 and 80 mph indicated on the speedometer and sure enough, when I converted the kph shown on the HVAC I was always exactly 10% slower than the speedometer indicated. Can't say as I appreciate Porsche lying to us about the speed of our cars, but it's good to know.
Thanks, guys.
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05-06-2009, 09:07 AM
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#15
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Registered User
Join Date: Jan 2009
Location: Arvada, CO
Posts: 229
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Quote:
Originally Posted by schnellman
I thought it was a joke when I read that your HVAC controls could be used as a diagnostic tool, and to tell you your true speed. But, I tried it on my '98 and was truly amazed to find it was true.
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Exactly how do you do this?
I would like to check.
BC.
__________________
Its not how fast you go, or how expensive your toys are.
Its all about how big your smile is at the end of the day that truly matters.
'98 Silver Boxster, '08 Ducati 848, '89 Honda Hawk GT, '89 Honda Pacific Coast
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05-06-2009, 09:34 AM
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#16
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There Is No Substitute.
Join Date: May 2007
Location: West Coast
Posts: 3,253
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__________________
1999 Ocean Blue Metallic Boxster - blueboxster.com
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05-06-2009, 12:44 PM
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#17
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Registered User
Join Date: Feb 2008
Location: Du Monde
Posts: 2,199
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Quote:
Originally Posted by JCL12
can somebody explain to me how our speedometers can read 2-4 high but the odometers will still be fine? I don't uderstand!
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With a computer controlled system, it's easy... count actual wheel RPMs, do the calculation, and the ODO is correct. Add the 'fudge' factor (safety margin in Porsche parlance) before sending the signal to the Speedo. It's not at all mechanical, even the analog needle is actually electro-mechanical.
Last edited by Lil bastard; 05-06-2009 at 12:59 PM.
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