Go Back   986 Forum - The Community for Porsche Boxster & Cayman Owners > Porsche Boxster & Cayman Forums > Boxster General Discussions

Post Reply
 
LinkBack Thread Tools Display Modes
Old 08-01-2012, 09:54 AM   #1
2001 Arctic Silver 2.7
 
tony_fury's Avatar
 
Join Date: Jun 2012
Location: Huntington Beach,CA
Posts: 310
Garage
Speedometer not accurate?

Anyone notice a difference between the car speedometer and GPS speed?

Since the installation of my Kenwood with Garmin GPS I noticed a 3 MPH difference between the speedo and the GPS.

Car speedo reflects 73 MPH and GPS reflect 70 MPH.
Car speed at 65 MPH, the GPS reflects 62.

Kind of curious if anyone else notice a difference?

I verified the proper tire spec's are accurate, and I know a GPS can be subject to speculation..
I also used the GPS on my iPhone and it reflects that same speed as the Garmin GPS..

T

__________________
2001 Boxster - Arctic Silver Metallic
RMS, IMS, 87.5K
tony_fury is offline   Reply With Quote
Old 08-01-2012, 09:55 AM   #2
Registered User
 
Join Date: Jul 2004
Location: Houston, Texas
Posts: 7,243
Yup, many of have noticed a 2-3 mph difference. The needle is always a bit higher.

Which is good for speeding tickets, or moe accurately avoiding them by accident.
RandallNeighbour is offline   Reply With Quote
Old 08-01-2012, 10:12 AM   #3
Registered User
 
mountainman's Avatar
 
Join Date: Jun 2012
Location: Little Switzerland, north carolina
Posts: 551
Garage
I have 2 boxsters and a C 4 and they all read from 2 to 5 MPH more than the gps.
mountainman is offline   Reply With Quote
Old 08-01-2012, 10:21 AM   #4
Registered User
 
Join Date: Nov 2010
Location: Philly
Posts: 594
No matter what car I am in and no matter what GPS I use, I get the exact same result.

GPS shows 2.5 mph lower than the speedometer.

I tried this test with a 2000 Honda Accord, 1996 Mazda Protege, 2003 Chevy Tahoe, 2012 Ford Focus, 2008 Toyota Rav4, 2001 Nissan Altima, and my 2000 Porsche Boxster. Same results in all of them.

I used my Garmin in all of them.

I tried TomTom in 3 of the cars mentioned above. Same result. GPS is 2.5mph slower.
__________________
00 2.7 5SPD MT
ProjectM96 is offline   Reply With Quote
Old 08-01-2012, 11:17 AM   #5
Certified Boxster Addict
 
Join Date: Nov 2010
Location: Los Angeles
Posts: 7,669
My speedo always reads 2-3mph faster than my GPS.

Unfortunately, I take this into account and go 2-3mph faster!
__________________
1999 996 C2 - sold - bought back - sold for more
1997 Spec Boxster BSR #254
1979 911 SC
POC Licensed DE/TT Instructor
thstone is offline   Reply With Quote
Old 08-01-2012, 01:45 PM   #6
Registered User
 
Join Date: May 2011
Location: Essex, CT United States
Posts: 301
Wildly inaccurate speedo - and I hate it. I'm always doing the math in my head, trying to figure out at what indicated speed am I pushing the limit of what Johnny Law will tolerate. I think it's just inexcusable - my Toyota is dead nuts to the GPS as is my Explorer. Yet my most expensive car can't get the most basic instrument correct. It's total BS. Makes you wonder if the tach is wrong too.
__________________
2002 Boxster S, Arctic Silver, 18" Turbo Wheels, 6-speed

Last edited by peterbrown77; 08-01-2012 at 01:59 PM.
peterbrown77 is offline   Reply With Quote
Old 08-01-2012, 02:52 PM   #7
Registered User
 
jb92563's Avatar
 
Join Date: Apr 2012
Location: Riverside, CA
Posts: 1,665
Yes, all of the speedos read Higher than actual depending on your tire diameter/size.
~3 mph faster.

It helps keep your speed down a bit and many times I have been glad it has as I cruise by an unseen CHP a little on the high side.
(Radar Detector ordered before its too late)

Your allowed 10% over actual by most police unless they are feeling ornery or are just plain bored.

They are generally not amused when you mention that due to the rotation of the earth you are already traveling ~300mph plus or minus your speed depending on direction.

If they offer to write you up for 300mph, accept, so that when they show up in court and cannot demonstrate to the Judge how they added or subtracted your cars speed vector from the 300, it will get thrown out.
Plus the judge will want a demo in your 300 mph Porsche
and you can frame the ticket and put it up in the garage as a conversation piece.
__________________
"It broke because it wants to be Upgraded "
2012 Porsche Performance Driving School - SanDiego region
2001 Boxster S, Top Speed muffler, (Fred's) Mini Morimotto Projectors, Tarret UDP,
Short Shifter, Touch Screen Dual Din Radio, 03 4 Bow glass Top (DD & Auto-X since May 17,2012)

Last edited by jb92563; 08-01-2012 at 02:54 PM.
jb92563 is offline   Reply With Quote
Old 08-01-2012, 02:55 PM   #8
Registered User
 
Bruce Wayne's Avatar
 
Join Date: Sep 2010
Location: UK
Posts: 303
Garage
in Europe, manufacturers must not allow a speedometer to under-read and the maximum permissable overread is by a calculation (this was discussed on a professional pilot forum i use).

by law in the UK we are allowed 110% +4

yes, i know it's europe, but the cars are designed and built to be sold across continents

for those interested in the legalities it's here: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:1975:196:0001:0005:EN:PDF

honda got drawn up on the over-read in US, here: Odo Uh-Oh: Honda extending warranties on 6 million cars
__________________
986 x2 6sp
2x Range Rover Vogue 4.6
2004 MX5 Sport 6speed Strato Blue (wifelet)
2x Range Rover Classic & CSK
Bruce Wayne is offline   Reply With Quote
Old 08-01-2012, 03:22 PM   #9
Registered User
 
Join Date: Mar 2012
Location: Arizona
Posts: 720
Quote:
Originally Posted by jb92563 View Post
Yes, all of the speedos read Higher than actual depending on your tire diameter/size.
~3 mph faster.

It helps keep your speed down a bit and many times I have been glad it has as I cruise by an unseen CHP a little on the high side.
(Radar Detector ordered before its too late)

Your allowed 10% over actual by most police unless they are feeling ornery or are just plain bored.

They are generally not amused when you mention that due to the rotation of the earth you are already traveling ~300mph plus or minus your speed depending on direction.

If they offer to write you up for 300mph, accept, so that when they show up in court and cannot demonstrate to the Judge how they added or subtracted your cars speed vector from the 300, it will get thrown out.
Plus the judge will want a demo in your 300 mph Porsche
and you can frame the ticket and put it up in the garage as a conversation piece.
You'd lose the entire court room if you tried to go into centripetal force and angular mechanics
Crono0001 is offline   Reply With Quote
Old 08-01-2012, 04:12 PM   #10
Registered User
 
bar10dah's Avatar
 
Join Date: Aug 2011
Location: Neither here, nor there...
Posts: 458
Quote:
Originally Posted by Bruce Wayne View Post

by law in the UK we are allowed 110% +4
I imagine 110% and +4 can only really truly happen at one speed. You can have your speedo read 110% faster, or you can have it read 4 MPH faster. But only at one specific crossover point, will both be true. Am I thinking about this correctly?
bar10dah is offline   Reply With Quote
Old 08-01-2012, 04:25 PM   #11
Registered User
 
bar10dah's Avatar
 
Join Date: Aug 2011
Location: Neither here, nor there...
Posts: 458
Also, Honda's odometers did (maybe still do) read 2-4% higher. However, their speedometers, at least all the ones I've driven, have been spot-on. I've had five Hondas now.
bar10dah is offline   Reply With Quote
Old 08-01-2012, 06:58 PM   #12
Registered User
 
MN 986's Avatar
 
Join Date: May 2008
Location: MN
Posts: 327
Garage
I have 2 Acuras that are dead on with the GPS, but the Boxster reads 2.5 to 3 mph over at all speeds. I figure if I drive 5 over the speed limit by the speedo, I am really only a couple mph over.
MN 986 is offline   Reply With Quote
Old 08-01-2012, 07:19 PM   #13
Registered User
 
gomarlins3's Avatar
 
Join Date: Jan 2012
Location: Kuna Idaho
Posts: 236
When my 986 was on the OEM 16" wheels and tires that came with the car, the speedometer read 5 MPH too fast. I switched to the OEM 17" wheels and tires that came on my wife's Cayman and my speedometer is right on now.
gomarlins3 is offline   Reply With Quote
Old 08-01-2012, 07:21 PM   #14
Theoretical propagandist
 
Eric G's Avatar
 
Join Date: Dec 2011
Location: Pacific Northwest
Posts: 793
Garage
With stock 17" I am +4 mph off from actual.
__________________
When life throws you curves, aim for the apex...
Eric G is offline   Reply With Quote
Old 08-02-2012, 12:33 AM   #15
Registered User
 
Bruce Wayne's Avatar
 
Join Date: Sep 2010
Location: UK
Posts: 303
Garage
Quote:
I imagine 110% and +4 can only really truly happen at one speed. You can have your speedo read 110% faster, or you can have it read 4 MPH faster. But only at one specific crossover point, will both be true. Am I thinking about this correctly?
at 30... 33 (110%) + 4 = 37

at 40... 44 (110%) + 4 = 48

at 50... 55 (110%) + 4 = 59

at 60... 66 (110%) + 4 = 70

at 70... 77 (110%) + 4 = 81

My Porsche speedometer over reads by close to 10%, at higher speeds the accuracy reduces, but the error seems linear. IE at 30 MPH indicated my GPS speed is 27 MPH and at 75 speedo indicated, the GPS speed is 70 MPH

Generally, the police will leave you alone on the UK M-way's at 80 MPH, so at 85MPH (speedo) i stay under the 10% error margin.

speeds high than that, no comment
__________________
986 x2 6sp
2x Range Rover Vogue 4.6
2004 MX5 Sport 6speed Strato Blue (wifelet)
2x Range Rover Classic & CSK
Bruce Wayne is offline   Reply With Quote
Old 08-02-2012, 05:33 AM   #16
Registered User
 
bar10dah's Avatar
 
Join Date: Aug 2011
Location: Neither here, nor there...
Posts: 458
Ah, okay. I misunderstood. I thought you were saying your speedo reads 110% of actual and +4 of actual at the same time throughout the entire range. I was tired last night when I read that...

Those are pretty big margins that the cops will let you get by with. Here, it's really up to the cop's disgression. He can write you a ticket for doing 1 MPH over. Or, he can let you go by at more than 20 MPH over. Just depends on who the cop is and what his mood is at that moment.

And, you're the first Boxster owner that I've read that has a speedo that's linear. All others report the same ~3-4 MPH higher reading throughout the entire speedo range. I wonder why yours is different? Stock wheel sizes?
bar10dah is offline   Reply With Quote
Old 08-02-2012, 06:49 AM   #17
SoK
Registered User
 
Join Date: Oct 2010
Location: NKY
Posts: 211
Going 73 as read from the digital readout in the dash, i'm going 69 - 70 by radar.
__________________
2001 Seal Gray 5-Speed - 2007 Yamaha R6 - 2003 Honda Civic EX - 1999 Nissan Frontier XE - 2003 Coleman Utah
SoK is offline   Reply With Quote
Old 08-02-2012, 07:53 AM   #18
Registered User
 
Join Date: Aug 2005
Location: Kansas City
Posts: 319
Based on the readings from my GPS, mine always varies by 5% at all speeds. At an indicated 80 the GPS reads 76, and at an indicated 50 the GPS reads 47-48, etc, etc. I've tracked this over many trips and it never changes. I just live with it. As was posted above, it's apparently a law in Europe and the fines for having the speedo read slow are, from what was discussed in a previous thread, pretty substantial. It's not that big a deal to me. If you can figure out what the factor is to correct for the error (assuming it stays constant, which it does on mine) it becomes pretty automatic when on the road.
__________________
79 911SC Targa.. gone but not forgotten
2001 Boxster Black/Savanah Beige
RoW M030 suspension
7/15 mm spacers
Deambered
Desnorked
SS door sills
Debadged
Clear tails
Technobrace
Technobra
I'll never own another black car!
Bob O is offline   Reply With Quote
Old 08-02-2012, 09:06 AM   #19
Registered User
 
Bruce Wayne's Avatar
 
Join Date: Sep 2010
Location: UK
Posts: 303
Garage
Quote:
And, you're the first Boxster owner that I've read that has a speedo that's linear. All others report the same ~3-4 MPH higher reading throughout the entire speedo range. I wonder why yours is different? Stock wheel sizes?
yep 18" as per the pics.. though i was tired when i wrote that too.
__________________
986 x2 6sp
2x Range Rover Vogue 4.6
2004 MX5 Sport 6speed Strato Blue (wifelet)
2x Range Rover Classic & CSK
Bruce Wayne is offline   Reply With Quote
Old 08-02-2012, 10:56 AM   #20
Registered User
 
Join Date: Jul 2011
Location: so cali
Posts: 302
Does this effect odometer reading as well?

ohhh my is offline   Reply With Quote
Post Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -8. The time now is 07:29 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Content Relevant URLs by vBSEO 3.6.0
Copyright 2024 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page