'05 GX470 Speedo Gone Wild -

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I was tagged recently on interstate 85 South in VA (speed limit is 70) going 87 MPH, but my cruise control was set at 72.  Took it to the dealer last week to get it checked out - they found nothing wrong, took it to get calibrated, they only found it off by 3MPH at a speed of 70MPH.

Driving it now with a GPS speed app, and it varies over long stretches.  Sometimes it is only by 5 MPH (i.e. GPS is telling me I am going 70, speedo shows 65), sometimes it is 10-12 MPH off - occasionally, the speedo is faster than the GPS (my wife said it was very erratic on her way home today).  These inconsistencies will often last for tens of miles at a time (i.e. 10-12 mph faster for a long stretch, or 5mph faster for a long stretch - it is not that it fluctuates wildly while driving - but there does seem to be some correlation to hard acceleration, also seems to "reset" itself when I go into park or turn the car off - and may be a different speed off when I start to run the car again).

1) First and foremost, I need to get the problem fixed - this is my wife's primary car and she is driving my two boys around all over the place.  I am working with the dealership (they have been good to work with so far) - but they seem to think it is a needle in a haystack issue that will be very difficult to diagnose and fix.  My independent mechanic (Toyota master certified and I have worked with them for over 10 years) basically said lexus speedometers are never wrong.

2) I need to fight the reckless ticket - the lawyer I contacted said my video evidence (had my bro in law video the GPS app next to the speedo while I was driving) would basically be inadmissible in court.

Has anyone heard of this type of issue before, and any ideas of how to fix it or what could be wrong?

Any guidance or advice would be greatly appreciated.



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I had a '05 GX before.

1) Firstly, I wouldn't rely on GPS readings, I would rely on speedometer (I think you are saying it has been checked). Secondly, what you described is perhaps just a normal operation: cruise control can't hit the brakes, it just stops hitting the acceleration when the speed starts to get closer to the set limit but if the road goes slightly downwards you will inevitably start running at a higher speed than set.

2) Even if admissible, GPS readings will likely be considered irrelevant as civil GPS has a tolerance + its accuracy depends on a number of environmental factors + reliability of GPS software is not a given, etc.

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Even if you can demonstrate to a court that your speedometer is defective, that is not going to get you out of a speeding ticket.  I suggest that you pay the fine and be done with it.

If you think your speedometer is acting up, consider sending your instrument cluster to Tanin for repair:  http://www.taninautoelectronix.com/

I've found the GPS speedometers in phone apps and in portable navigation systems (e.g. Garmin) to be incredibly accurate and I pay more attention to them than to in-dash vehicle speedometers.  When I drive, my cell phone is in an iOttie holder high on the dash displaying the GPS speedometer in the iBolt Dock'n Drive application.  Dock'n Drive even starts automatically when it senses that my phone has made a Bluetooth connection to our vehicle in-dash systems and keeps the phone screen from "timing out".  Dock'n Drive is an application organizer that makes applications safer to use while driving - its speedometer is a bonus feature.

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I have found the GPS to be extremely accurate, more so than most speedometers.  Now way could the car go 87 when the cruise is on 72, plus you would probably notice you were running faster than 72. Of course you should be using the WAZE app. there really is no need to be bagged so easily anymore. Good luck with the ticket.  Wayyy too bad you got this in Virginia where (for whatever reason) over 80 is reckless (which nit isn't of course) so you better lawyer up and get it reduced. Best of luck with you on this.

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Hi All, I am back to the forum (from another computer after a few weeks due to the crash of my 'master' laptop).

1990LS400, VBdenny, It's interesting. I found no proper topic on this forum, so replying here.

Question: Do you find car speeds reported by consumer GPS devices to be reliable/accurate (at all; or yet even more reliable than the speed reported by your stock speedometer)? If so, why?

This is no trolling, I really got interested in the subject. I personally almost always use my phone for GPS routing and car speed readings in unknown places. I am used to seeing GPS speed on the phone (so conveniently attached to the windshield just slightly below the eyes level). I am similarly used to seeing GPS readings' digits updated so often. These two factors have led me to (subjectively) perceive GPS speed readings as 'being reliable': from my 'vacation memory', in most of the cases when I've used GPS, I've also relied on its speed readings, not bothering myself to look at stock speedometers too often. 

As it's an interesting subject, I've done a bit of a research on the web about a month ago. Some highlights of what I've found (unfortunately, all that I'd downloaded/saved was lost with the crash of my laptop -- I hope the laptop is recoverable but It'd take some time to recover it). From memory:

(1) Requirements as to stock speedometers:

(1.1) are different in various countries. US law for commercial vehicles requires them not to read below the actual speed but allows a higher-speed reading of, from memory, up to 10%. I found no US federal requirements for non-commercial vehicles like which we own for domestic use;

(1.2) in Europe (largely, the EU), a +/- 10% (maybe a higher number? 12%?; can't remember) tolerance is allowed;

(1.3) in my research at the time, I mainly focused on the USA, less on the EU. However, Australian rules may be different. Theirs may be upside-down :). Web reports of when one was able to override speed tickets by referring to GPS speed readings are mostly related to Australia;

(1.4) I think I've seen some similar speed-override reports from the States, but all were related to commercial- but not consumer-type GPS systems (i.e. you probably need to buy a GPS commercial subscription to really be able to refer to their GPS speed measurements in court -- see below).

(2) Requirements as to GPS speed readings:

(2.1) found no rules (not just any 'unified' rules - no rules at all).

All sources that I found at the time (including what I took as an 'official' GPS source) were largely oriented at GPS positioning rather than speed measurements. All sources found referred that the GPS reliability is (a) subject to its design tolerance levels and (b) exposed to various environmental factors. Civilian GPS (like one we use in smartphones) is the least reliable of the GPS systems; military GPS is the most reliable (but still has its design tolerance level). In the middle, corporate GPS systems seem to be reliable enough so that their readings are sometimes accepted by courts as admissible+relevant evidence -- though, I've failed to find as to which particular level they may be more reliable than the civilian GPS. Examples of corporate GPS are, e.g., paid subscriptions to monitor your trucks/buses' speed and location.

(2.2) having not found any sensible rules for GPS speed measurements, I started to look for scientific studies. Same as above, found a lot related to GPS positioning but only really a few reports related to GPS speed measurements. One of the studies I've found got some of my trust as the guys looked for GPS speed readings in the 'real world' environment and from various angles e.g. they tested GPS speed measurements from within a car, driving it from highways to city roads, from straight to curvy roads, and (from memory) built some other interesting factors into their analysis. Interestingly, from memory, they concluded that GPS (as far as it's speed measurement is involved) was ~20% accurate.

I am feeling very sorry that I cannot provide proof-links or attachments. All the analysis has been performed on my laptop which has crashed (I expect that my laptop's memory is recoverable -- though it may take some time).

In the interim, any further thoughts on the 'question' (repeated below) are welcome -- to me, this is a very interesting subject. When replying, please try to avoid the subjectivity/feelings and rather focus on facts.

Question: Do you find car speeds reported by consumer GPS devices to be reliable/accurate (at all; or yet even more reliable than the speed reported by your stock speedometer)? If so, why?

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