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Old 08-01-2012, 09:54 AM   #1
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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
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Old 08-01-2012, 09:55 AM   #2
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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.
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Old 08-01-2012, 10:12 AM   #3
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I have 2 boxsters and a C 4 and they all read from 2 to 5 MPH more than the gps.
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Old 08-01-2012, 10:21 AM   #4
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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.
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Old 08-01-2012, 11:17 AM   #5
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My speedo always reads 2-3mph faster than my GPS.

Unfortunately, I take this into account and go 2-3mph faster!
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Old 08-01-2012, 01:45 PM   #6
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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.
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Old 08-01-2012, 02:52 PM   #7
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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.
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Old 08-01-2012, 02:55 PM   #8
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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
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Old 08-01-2012, 03:22 PM   #9
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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
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Old 08-01-2012, 04:12 PM   #10
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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?
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Old 08-01-2012, 04:25 PM   #11
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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.
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Old 08-01-2012, 06:58 PM   #12
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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.
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Old 08-01-2012, 07:19 PM   #13
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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.
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Old 08-01-2012, 07:21 PM   #14
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With stock 17" I am +4 mph off from actual.
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Old 08-02-2012, 12:33 AM   #15
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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
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Old 08-02-2012, 05:33 AM   #16
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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?
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Old 08-02-2012, 06:49 AM   #17
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Going 73 as read from the digital readout in the dash, i'm going 69 - 70 by radar.
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Old 08-02-2012, 07:53 AM   #18
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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.
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Old 08-02-2012, 09:06 AM   #19
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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.
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Old 08-02-2012, 10:56 AM   #20
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Does this effect odometer reading as well?
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