Speedo difference.

Submitted: Sunday, Aug 04, 2013 at 16:07
ThreadID: 103562 Views:2155 Replies:10 FollowUps:19
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My speedo is a little out according to the GPS. Speedo says 100, GPS, 95.
Does this mean the trip meter is also out? Driving an NP Pajero.
Thanks.
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Reply By: Member - Boobook - Sunday, Aug 04, 2013 at 16:13

Sunday, Aug 04, 2013 at 16:13
Probably not, the speedo is usually set to about 3% slow and odometers are generally spot on.



AnswerID: 515894

Follow Up By: Ross M - Sunday, Aug 04, 2013 at 18:03

Sunday, Aug 04, 2013 at 18:03
BooBook

It depends on your point of view.

I reckon all speedos are actually set higher than true speed, not low, so you have a margin of insurance. By law they are more, not allowed to be less.
Often reading up to 10% more.

I do agree most Odometers are pretty close, but they too do vary with the various sizes of tyre approved for normal fitment.
The largest of the tyres recommended will be covered by the over indication error.

Ross M
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Follow Up By: Member - Boobook - Monday, Aug 05, 2013 at 05:50

Monday, Aug 05, 2013 at 05:50
Ross, you are right, I didn't phrase it very well. They read faster than actual speed as you say.
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Reply By: Lex M - Sunday, Aug 04, 2013 at 17:59

Sunday, Aug 04, 2013 at 17:59
Why not check it against the GPS. Then you'll know.
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Reply By: Member - Alan H (QLD) - Sunday, Aug 04, 2013 at 18:12

Sunday, Aug 04, 2013 at 18:12
GPS in a straight line is quite accurate compared to speedo.

Alan
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Follow Up By: Member - PJR (NSW) - Sunday, Aug 04, 2013 at 18:49

Sunday, Aug 04, 2013 at 18:49
True Alan

For GPS speeds: also not all roads are straight so they can be out heaps in the mountains and on windy roads. Next time you go around a long tight "U" bend the GPS speed will be adjust according to the previous calculation. I have seen speeds of 300KPH (great Toyota we have hey!!!) in the actual data files (saved track files). But when you replay them you in a replay you will see they mostly stay as per the previous one. GPS speeds are calculated every set straight line distance. Hence the errors.

We all know how car speedos work so it is easy to get a difference both ways.

I trust both and tend to believe the accuracy of the GPS only on dead straight roads as Alan notes.

Phil
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Follow Up By: Ross M - Sunday, Aug 04, 2013 at 19:08

Sunday, Aug 04, 2013 at 19:08
Phil
Does the GPS also work on roads with no wind?

Do we all know how a car speedo works?

The GPS uses time intervals and if going around a curve then the longer time to get between two points is therefore indicated as a slower than normal speed, can't see how it can be faster.
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Follow Up By: SDG - Sunday, Aug 04, 2013 at 19:37

Sunday, Aug 04, 2013 at 19:37
I got no idea how speedo works.
You lost me on the pulse thing mentioned below.
Last time I looked at speedo workings, was when I was on my p's, driving my mothers car, with the cable disconected to the odometer/speedo, so that she would not know how far I travelled in her car with my mates.
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Follow Up By: Ross M - Sunday, Aug 04, 2013 at 20:22

Sunday, Aug 04, 2013 at 20:22
SDG
For a quite a long time there have been no cable driven speedos in common use. All have an electronic pulse unit which creates pulses in proportion to the tailshaft rotation.

This is then counted as "How many" for distance and "How fast" for speedo use. Although both units are on the dash somewhere, they are different units where years ago they were integrated into one unit although they were really a two in one anyway. They have to be different or they would both be Speedos or Odometers.


The drive cables have been replaced with electrical cables.
Ross M
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Follow Up By: Member - PJR (NSW) - Sunday, Aug 04, 2013 at 22:12

Sunday, Aug 04, 2013 at 22:12
And another oops for me Ross. I thought that it was so simple. A pulse or cable from the drive train turns gears or electronics in the speedo or dash computer display to show the speed.

Ah well What has this advancement in technology done to basic knowledge!!!! Next they will have to Google "How do you fix a leaking tap" to fix it.

Anyway enough Phil!!!

But I did find a data record saying once that the car had travelled at 300KPH for 10 seconds. David and I had a go at figuring it out. The profile plot of distance against speed also showed the huge spike to 300. Very strange. We are taking our time in getting a replacement windscreen safety cam because of this error. Just imagine. you are showing the police the log to prove that you weren't speeding and he sees a huge 100K+ spike. Whoops. It would also most likely make it insubmissable at court.

Phil
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Follow Up By: SDG - Sunday, Aug 04, 2013 at 22:26

Sunday, Aug 04, 2013 at 22:26
Keep that record PJR. Could come in handy when time to sell vehicle. Some young gullible revhead might think he is getting a sports car capable of offroad.....lol
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Follow Up By: Ross M - Monday, Aug 05, 2013 at 10:52

Monday, Aug 05, 2013 at 10:52
PJR Phil
If your GPS lost satellites for a short time and then regained the signal feed the GPS will see it as large distance covered in a short frame of time and since the time and distance ratio has been interrupted it may show it as a sudden increase in speed. Did you feel the sudden push back into the seat?

Is the position reference near, under or alongside powerlines?????? They often blot out the signal.

Cheers
Ross M
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Follow Up By: Member - PJR (NSW) - Monday, Aug 05, 2013 at 14:33

Monday, Aug 05, 2013 at 14:33
Ross, you may well be right.

It was on the Snowy River Road just north of the Ingegoodbee Track intersection in east Gipsland. A hilly and treed area.

Phil
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Follow Up By: Jarse - Tuesday, Aug 06, 2013 at 19:43

Tuesday, Aug 06, 2013 at 19:43
You can also get errors either way as different satellites in the constellation become visible (and used) over existing ones.

Automotive GPS is *reasonably* accurate, but I wouldn't depend on its readouts in anything but a prolonged straight line on level ground.
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Reply By: Iza B - Sunday, Aug 04, 2013 at 18:26

Sunday, Aug 04, 2013 at 18:26
My speedo reads 108 k/h when my GPS tells me I am actually travelling at 100 k/h; the speedo overreads by about 8%. Incidentally and typically, the overread is within ADR requirements.

My odometer tells me I have travelled 103 k when GPS devices tell me I have traveled 100 k.

In most cases, the speedo and odometer calculate things a little differently so I suggest you do a little test to establish the error in trip meter readings as they may very well be different to the speedo error.

Iza
AnswerID: 515904

Follow Up By: Ross M - Sunday, Aug 04, 2013 at 19:16

Sunday, Aug 04, 2013 at 19:16
Iza B

Modern speedos work on pulse rate in unit time to indicate speed.
IE "how fast do we have to count".
Odometers may use the same pulses source but they count pulses as a preprogrammed distance and indicates that in a cumulative manner. IE "how many have I got".

Two entirely different items.
The 3% is usually in the odometer to allow for the larger sizes tyres which may be used on bigger rims but are both on the tyre placard of the vehicle. All mentioned sizes therefore fit within the error. May be no error if tyres are a tiny bit bigger than the other.
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Reply By: Les - PK Ranger - Sunday, Aug 04, 2013 at 18:57

Sunday, Aug 04, 2013 at 18:57
With new tyres, my speedo is 3km/hr under actual speed, when the tyres are 95% worn, it's more like 8km/hr over.
I think it's pretty much the norm all round now with recent model vehicles to err on the side of caution.

Better than being the other way round I suppose, and why I think many states have now dropped speed detection tolerances to a couple of km over.
In a 60 zone, they know when your done doing 62, your speedo is probably showing 65+.

I've got to admit, never have checked odo to distance, presumed it would be out the same margin, but maybe closer to actual, at least with newish tyres.
AnswerID: 515907

Follow Up By: Ross M - Sunday, Aug 04, 2013 at 19:25

Sunday, Aug 04, 2013 at 19:25
Les
To have 11% difference in the indicated the tyre would have to wear from
say a 245 x 16 x 75 down to around a 245 x 50 x16 for that degree of effect 20% in aspect ratio and is more than a tyre tread depth.
Almost 100mm in diameter of wear.. I think the -3% up to +8% is not quite right somehow.
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Follow Up By: Les - PK Ranger - Sunday, Aug 04, 2013 at 21:13

Sunday, Aug 04, 2013 at 21:13
Sorry, my mistake typing, that should have been when new reads 3km over on speedo to actual.
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Follow Up By: Jarse - Tuesday, Aug 06, 2013 at 19:37

Tuesday, Aug 06, 2013 at 19:37
Should be the other way around, Les. As your tyres wear, the circumference gets smaller, hence your speedo should progressively overread your true speed. Your indicated speed always remains the same, but your true speed gets slower as your tyres wear.
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Follow Up By: Les - PK Ranger - Tuesday, Aug 06, 2013 at 19:45

Tuesday, Aug 06, 2013 at 19:45
As your wheels turn faster with tyre wear and increasing smaller circumference, the speedo reads higher, ie instead of 3ks over true speed with new tyres, when worn to around 95% tread I find the speedo reads about 8ks over true speed.
I think that's what I typed, new tyes 3km over true, well worn 8ks over true.
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Reply By: Nomadic Navara - Monday, Aug 05, 2013 at 00:11

Monday, Aug 05, 2013 at 00:11
Speedometers are governed ADR-15. Search for it to get the complete PDF file. The following is the section on speedometer calibration

PeterD
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AnswerID: 515930

Follow Up By: Nigel Migraine - Tuesday, Aug 06, 2013 at 17:52

Tuesday, Aug 06, 2013 at 17:52
Goodness! A man who follows logic, reason, investigation and the production of evidence! What are you doing on this forum?
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Reply By: Barbera72 - Monday, Aug 05, 2013 at 09:16

Monday, Aug 05, 2013 at 09:16
While driving on Bruce HWY: speedo reading 105, garmin handheld GPS 100, my brother's mobile phone GPS 95. :-)
AnswerID: 515937

Reply By: disco driver - Monday, Aug 05, 2013 at 15:05

Monday, Aug 05, 2013 at 15:05
Does it really matter?
It lessens the chance of picking up speeding tickets and the difference in distance, if you're worried about odometer readings being inaccurate just means that your services will be about 30-50 km sooner on a 10,000km service schedule.
A none event in my opinion.

Disco.
AnswerID: 515946

Follow Up By: landseka - Tuesday, Aug 06, 2013 at 06:56

Tuesday, Aug 06, 2013 at 06:56
If the odometer is up to 10% out as are a lot of speedos then the service interval could be as much as 1000 kms too early on a 10,000km schedule, not 30 - 50km.

The dealers would love this.
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Follow Up By: disco driver - Tuesday, Aug 06, 2013 at 11:08

Tuesday, Aug 06, 2013 at 11:08
OK, Granted, my maths were never my strong point, but even so in the scheme of things an early service is better than a later one.
You would have to pay for 1 more service (say $1000 max) in 100000km.
Not enough to really be worried about, is it?

Disco.
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Reply By: olcoolone - Monday, Aug 05, 2013 at 16:55

Monday, Aug 05, 2013 at 16:55
WOW..... 21 posts just to tell him it's normal, lucky he didn't ask something technical.
AnswerID: 515955

Reply By: Gado - Tuesday, Aug 06, 2013 at 21:15

Tuesday, Aug 06, 2013 at 21:15
With current tyres, my speedo says 105 when true speed is 100. True speed is based on stopwatch (30 sec for measured kilometre = 120 km/hr, 36 sec = 100km/hr etc). This means speedo is about 5% FASTER than actual at around 100km/hr.

GPS readout speed (Garmin) is also confirmed very accurate by this method.

At the same time my odometer reads about 2% LESS than actual kms, based on many comparisons with maps and GPS over long trips. Apparently no connection between the speedo and odometer!

(Hyundai Terracan)

Cheers, Gado
AnswerID: 516027

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