Does anyone out there face the problem that the speedo meter reading is different from the GPS speed reading? Like when your speedo meter reading is at 110km/h, the GPS reading is only at 105km/h or already over the speedo meter reading like 120km/h?

GPS is use satellite to calculate your location and speed. Speedo meter from your car is calculate direct from your tayer turning.
2 difference calculate but speedo meter more correct then GPS.

A slight correction here. Speedometer calculation is done from gearbox turning and not and not really tyre turning. That is why if you change tyres to the wrong OD from OEM, the speedometer is no longer accurate as the manufacturer has tuned the speedometer to match the factory tyres.

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Added on August 1, 2008, 3:33 pmThere is a 2 second delay for signals traveling from ground level to the satellites (and this is via big sat dish, would be longer for small GPS devices). The data will no longer be accurate.

The only accurate way to measure your speed is to use a speed camera. This gives you your Actual Speed. The next level of accuracy would be to use your speedometer. This gives you your Metered Speed.

Using GPS to track your speed will not be accurate as there is a latency in your GPS signal that would interfere in the calculations of your speed.

This post has been edited by sleepwalker: Aug 1 2008, 03:33 PM

Nope, GPS is more accurate.

And that's why our PDRM allow a 10km/h difference b4 sending u a speeding ticket.

Ya, I heard people said GPS is the most accurate way to know your vehicle speed. If not, satelite will cause a lot of error and all the telecomunication including Astro won't able to work properly liao because it is using digital.

That's where you are wrong. PDRM does not give you 10km/h difference. That's because they are capturing your speed from an angle. For speedo cameras to be accurate, it has to be shot directly in front of the car. Any angle will give you a variance. So actually, they are not giving 10km/h difference, it is just that they are always capturing a slightly slower speed than your actual speed. Furthermore, in most cars, there is a meter variance of up to 5% faster, meaning that your speedmeter is faster by up to 5% at 110km/h.

This combined with your actual speed taken at an angle, people end up with the urban myth that the police give you 10km/h more before saman. That is wrong because if you are travelling at about 120km/h on your speedometer, your actual speed is most probably around 115km/h with the police capturing your speed at 110km/h.

JUST USE UR TACHYMETER...

Thanks for correction..
But my GPS speed lower then my speedo meter.
I using garmin Q. hehehe

This post has been edited by LeVis_Jeans: Aug 1 2008, 03:56 PM

I'm not talking about error. I'm talking about lag. GPS speed calculation works by calculating your distance traveled divided by time. However, due to differences in the lag time that it take for the signal to get to the satellite. Since the lag time varies as you move, it affects the real time calculation of your speed.

Astro works fine with sats because they are streaming a constant rate to earth. It is not affected by the lag time. Have you tried internet communications over Sats? I have tried it from our very own Earth Satellite station in Bukit Jalil. Signal takes a minimum 240ms (average about 500ms) to get from earth to the sat (one way) and another lag of 240ms-500ms from Sat to earth.

Ok, I would not say that GPS is not accurate but rather it is not precise.

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Added on August 1, 2008, 4:00 pmThat's because your metered speed is always faster than your actual speed. The manufacturer's made it this way.

This post has been edited by sleepwalker: Aug 1 2008, 04:00 PM

Yes.. strong agree with him.
GPS lag.
GPS is using a device send keep send signal from your location to satellite, then receive signal from satellite and your device show which location you rite now.

Speedo meter from GPS is calculate how many meter you already pass per second.
Remember is how many meter already pass..

I am using GMXT and the speed reading in the GPS is same with my car speed.

If the police captured my speed at say 1km/h over 110km/h, then why the record on the saman or the picture taken will record 120 or 121 km/h?

before u go on ranting bout GPS, do u even understand how it works ?
what type of GPS are u talking about ? aGPS ? or plain 1990's GPS ? HSGPS ?

and no, the GPS machine you have isnt accurate. whether its on a crummy i550 or N95 or garmin nuvi or tomtom or mio c230 or magellan its not accurate.
if it is, please head on over to mypdacafe.com and there'll be 1000 users waiting to buy the system from you to sell to the military. even military's high sensitivity receivers condones a .7 sec delay.

aGPS 7 HSGPS uses calculation correction methods to make it seem like its on real time but it just isnt. so u may be travelling 100km/h @ 12.09.0543am but in actual fact u're not.

like what sleepwalker has said, if u wanna know what speed u're at, on that current moment, use a speed camera. they exist for a reason.


btw there was once a court case hearing from a police officers son, who claimed the speed on his sat nav didnt tally with the speed camera captured. the judge ruled it out and arguments went on.
anyway the speed camera recorded the real deal and the judge ruled it in favor. i'll see if i can dig up that old news. it came out in the newspapers once in the star. forgot when.

This post has been edited by sharkteef: Aug 1 2008, 05:08 PM

whatever your actual speed is.

what speedwalker pointed out is that speedometer reading and actual speed (laser speed trap camera or old fashion radar gun) might not be the same and are subjected to calibration tolerances.

I have tried 4 different brands GPS at once and the result is the same, all also shown the same speed, but maybe some will a little bit slower, but not much, maybe less than 1 second, this is probably due to the processor speed, so I think the GPS is quite accurate kua.

and i highly agree with the words that u used. quite accurate is right.

How do you know that you were traveling at 111km/h and that they captured and recorded you as 121km/h? I don't quite get your point.

I think it depends on the GPS unit as well. I'm using a Garmin Nuvi and the speedometer in the GPS is very accurate when I'm travelling back to Ipoh from KL at highway speeds. I believe if tested on a windows mobile phone cum GPS, there's a noticeable lag.

I was just about to mentioned about crummy GPS systems aka RM2k types but you beat me to it.

I was trying to tell them about the lag as even BIG SAT DISH cannot beat the laws of physics. Fastest turn around time one way is about 240-250ms from earth to Sat. Combine that with the triangulation required, even the Military's super duper GPS will have a lag of 0.7 secs turnaround time as you mentioned. Some just don't understand and think that everything travels in the speed of light. Unfortunately, Sat comms signals do not travel at the speed of light.

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Added on August 1, 2008, 6:08 pmThat is why I mentioned earlier, it is not about the accuracy but rather the precision. The GPS would most probably be accurate if you travel in a constant speed in a constant direction as it can easily calculate the distance traveled and provide a more precise calculation. But once you start to slow down, change speed and direction, the calculation will not longer be so precise.

This post has been edited by sleepwalker: Aug 1 2008, 06:08 PM

you're welcome !

adding on...... nothing is real time ! not even calculations (based on moving vehicle because when u're stagnant why would it need to be real time anyway). if sat navs are all in real time + accuracy, our body would've been mutated by radiowaves and we'd all have cancer. i dont want cancer.

i'm driving myvi ezi..last week using my bro's n82 gps..the reading is almost exact.

Fantastic thread! minus the comments from users which does not read and simply post based on their own findings, not facts.. ah sometimes ignorance is a bliss..

same to you, because you bump an old thread.

Agree, I drive kancil, the GPS reading are same as the speedo meter. When I drive the old saga, which have about 10km/h faster then the GPS. Its even worst for motorbike, about 15-20km/h different.

Conclusion that Perodua had the most accurate speedo meter. So don't speeding, there is not tolerate variance for Perodua driver.

i agree, example, if you have 2 set of tv, try to open same channel, let say, tv3 using normal and tv3 using astro, you can see that the tv3 using astro is slightly delayed

This is because your car is still in stock, nothing change, especially rims or tyres.

i got warning before from police...speed limit there was 80...
on my meter shows 110.the police showed i was speeding about 123kmh..almost the same like ur theory..default rim was 14 ...changed to 15

This post has been edited by xshiro: Apr 2 2009, 07:29 PM