Speedo difference.

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

Ross, you are right, I didn't phrase it very well. They read faster than actual speed as you say.

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

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.

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.

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

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

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

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

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

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.

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.

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.

Sorry, my mistake typing, that should have been when new reads 3km over on speedo to actual.

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.

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.

Goodness! A man who follows logic, reason, investigation and the production of evidence! What are you doing on this forum?

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.

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.