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Master Geek


Topic # 55248 26-Dec-2009 22:35
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hello there i got just a small question for you guys about speed accuratey in cars.
i got a tom tom xl340 IQ Routes Edition it displays my speed when im traveling in my car it says im only doing 46 when my speed o says im doing 50ks. so i check the GPS accuratey aginst the speed signs. that you see on that side of the road that say 'Your Speed Is'. and the GPS will say the same speed as the speed sign that works by radar. now when im on the highway in the 100k zone my speed o will say im doing 110k but my GPS will say im doing only 100.

what one should i go by the GPS or the speed o?

[Moderator edit (MF): edited spelling on subject]

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  Reply # 285321 26-Dec-2009 22:47
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Neither is guaranteed to be accurate. Your GPS works by calculating your speed from the distance travelled over a given time which as you can tell from the sometimes oddball guesses it gives you on your current location when it can't get a fix (what do you expect, when it's calculating your position from orbit?!?), the speedo is more integrated into the car but there is a possibility of a variance between what it shows and your actual speed (incidentally, this is the reason for the historic 10Km/h "grace" from the speed limit).

Really, you're probably safer going by the speedometer. If you get caught going 115Km/h because the GPS says you're going 105Km/h and the speedo says you're going 115Km/h then I can guarantee the police won't accept "but my GPS says I was going slower" as an excuse.




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  Reply # 285323 26-Dec-2009 22:48
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GPS, speedo's are always out, depending on:
- non-factory tyres
- non-factory mags
- flatspots on tyres
- speedo needle wrong
- speedo not calibrated
- speedo cable kinked
- speedo cable end about to break

lots of other combinations, try a speedo calibration place and they should be able to test it




I have moved across the ditch.  Now residing in Melbourne as a VOIP/Video Technical Trainer/Engineer. 

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  Reply # 285324 26-Dec-2009 22:48
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speedometer is usually overestimating. i cant say about GPS whether it under or overestimates. you really want to be safe and go somewhere in between.

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  Reply # 285325 26-Dec-2009 22:50
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whoops replied at the same time :)

yeah both are probably inaccurate, I'd say speedo would tend to me more inaccurate.

Best not to risk speeding on either ;)




I have moved across the ditch.  Now residing in Melbourne as a VOIP/Video Technical Trainer/Engineer. 



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Master Geek


  Reply # 285328 26-Dec-2009 22:58
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i don't want to speed i just wanna go the 100 k in the 100 zone. id like to say the GPS does well considering its using satalight from outa space to get my speed. and then there's the stalker radar that the nz police use whats that going to say?

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  Reply # 285330 26-Dec-2009 23:01
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Personally I would probably say the GPS is better because the road-side speed displays always are exactly what my GPS device says every time.

General note: Before I changed my stock wheels to mags and my speedo was 108km/h and my GPS was saying 100km/h, after I upgraded stock wheels to mags when my speedo was saying 101km/h my GPS was saying 100km/h so there are many variables such as tire size that alter the car's speedo reading as Kyanar says.

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Ultimate Geek


  Reply # 285331 26-Dec-2009 23:02
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Kyanar: (incidentally, this is the reason for the historic 10Km/h "grace" from the speed limit).


I always thought it was a 10% 'grace' rather than 10km/h. My understanding was anything over 55 in a 50 zone, 66 in 60, 88 in 80 and of course 110 in a 100 zone.

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  Reply # 285345 26-Dec-2009 23:49
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To be honest patatrat, I don't think either are codified in law - so I can't be sure of the specifics.




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  Reply # 285346 26-Dec-2009 23:49
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where would one get a speedo calibrated in auckland?

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  Reply # 285372 27-Dec-2009 08:01
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As noted before... GPS is an average of previous readings - not your actual speed but very close. The speedo on your car will vary depending on many factors.

Don't speed.




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  Reply # 285375 27-Dec-2009 08:55
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Out in the country the AA has placed signs where you can check your odometer, usually it's about 3km. You can use this to measure your exact speed, assuming you can drive at the same speed of course (pretty easy). These however are hard to find, you might want to use the mapmyrun.com which gives a lot of detail, you just choose a straight road and time between intersections.

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  Reply # 285391 27-Dec-2009 11:03
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I also have the same "problem" with a discrepancy between my GPS and car odometer. In my case it is less severe, with the car showing 100 km/h while the GPS indicates 95 km/h (5 % difference), this seems to scale linearly thus at 50 km/h the discrepancy is less.

For the odometer, on a recent trip between Bombay and Auckland the car trip reading was 109.9 km, whilst the GPS trip log was 107.5 (2.2 % difference).

I suspect that the GPS reading is more accurate, for the reasons articulated by vinnieg. I also don’t think it is coincidence that we all seem to have car speedometers that underestimate speed, I suspect car manufactures fear litigation if the reverse were produced.

Regarding the speed limit “grace” I can’t find any legislation dealing with this but here it says that the current Police speed policy specifies that:


  • Drivers of vehicles other than heavy motor vehicles who exceed the speed limit by less than 11km/h will not normally be issued with an infringement notice providing their speed does not present a risk to public safety in the prevailing circumstances.



  • These principles do not constitute a mandate for drivers to exceed the speed limit. In some circumstances exceeding the speed limit by less than 11km/h will pose road safety risks.




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  Reply # 285393 27-Dec-2009 11:07
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GPS is more accurate.
But, you could accelerate and slow down between point A and point B with a GPS reading at each point and it will give you one speed.

Here is a tyre size calculator that shows your speedo difference between tyres. Enter your stock size to compare with your current.
http://www.miata.net/garage/tirecalc.html

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  Reply # 285400 27-Dec-2009 11:54
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Police will not use GPS as a recognised speed detection/measuring device. So, if the Police car has a GPS unit, they will still use the speedometer, to measure your speed, while pacing, and issue an ION based on their speedo. If you were to argue that your GPS said otherwise, you would be required to go to court to prove that it is/was more accurate at that time.

As far as tolerance goes, there is no official figure. It is the Police officers discretion to issue a ticket when the speed is marginally over the posted limit. They will factor in road conditions, weather conditions and traffic etc There is zero tolerance in school zones.

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  Reply # 285733 28-Dec-2009 22:42
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The alarming thing is that the odometer is also usually out by the same amount, so you can easily be paying 10% too much in RUC if you swing that way, and servicing your car more often etc...

Funnily enough the speedo place that a friend took his ute too after putting bigger wheels on it said most people dont get the odometer changed when going to larger wheels ;)




Richard rich.ms

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