Speedo error margin (Part II)

[Taka]

I guess I open another thread because I have found something very interesting about our Kluger. Firstly I introduce my kit to measure the speed data of the car.

(click for large pics)

The left is the bluetooth GPS receiver (Qstarz BT-818 extreme). This is one of the best we can get without breaking my bank. This unit contains a 5Hz MTK chipset and can sample a GPS location every 0.2 sec. So it is 5 times as fast as my Toyota Grande navi system. The right is the new toy , and it is called OBDkey. OBD (on board diagnositc) system is used in all fuel injection cars. The unit can be used to see if any error code from the car recorded in the Electronic Control Unit (ECU, aka the brain of the car). It can show loads of data from the ECU, including real speed from the wheel (OBD speed), rpm, throttle position, fuel trim, timing advance, coolant temp, intake temp, catalyst temp etc etc etc. And the key is not Toyota or any car specific (work in 99% of cars :) )

I use both to work with my Nokia N95, using a program called racechrono.com.

It was a cold night (6 degree) so the power will not be as good, so I ditched the idea of setting the best 0-100 times. The above three graphs show the most revelant data in terms of the real speed that ECU recorded. Ignore the top lines in the first picture. I did a long run from 0-100 so I can match some nice figure. What you can see:

The PURPLE line is the OBD/ECU speed, while the YELLOW line is GPS speed

Observations

1. OBD shows speed updates in a very very slow rate (The OBDkey able to get 4Hz data, or 4 times a sec, but Toyota ECU can only give 2Hz at most )

2. The OBD speed is VERY close to the GPS speed. In fact, the OBD speed is LOWER than "real" GPS speed 90% of the time. When ECU is showing 60kmh, GPS is also 60; but when ECU is 97 kmh, GPS is already 99.9 kmh! <_<

:o BUT!!! :o

3. When the needle of the speedometer on the dash is just right to the 100kmh mark (as said by RiceRacing that is the "real" 100kph). The OBD/ECU speed is only 92kmh and GPS is 93kmh!!!!!!!

Conclusion:

Kluger Odometer analysis: ECU speed is very close and possibly slightly lower than real GPS recorded speed. And the error shown is mainly on the needle on the dash.

Is there any solution? Yes you can put a OBD display gauge in the car and use the speedo on it, but you have to realise they are very accurate and you have no margin for error (in terms of speeding)!

BTW: This is my try hard run for 0-100. Sometimes the OBD or the GPS lacks and create an error in timing, but looking at the graph, I am actually doing a 7.92 sec !

Edited July 19, 2008 by Taka

[holmesie]

Time to pull my Scanguage II OBD reader out of the box I think ;)

[PaulG]

Just a note:

Beware anyone buying the BT Q-818 gps receiver. It is not capable of 5Hz update times, only 1Hz. I found this out the hard way, by jumping in and buying one first before doing the research into it's real capabilities. The BT Q-818 EXTREME is the one you want.

Questions for Taka:

Is the ODB Key a bluetooth device? How much and where from?

How do you do the neat screen images from your phone?

[nerdygirl]

1. OBD shows speed updates in a very very slow rate (The OBDkey able to get 4Hz data, or 4 times a sec, but Toyota ECU can only give 2Hz at most )

2. The OBD speed is VERY close to the GPS speed. In fact, the OBD speed is LOWER than "real" GPS speed 90% of the time. When ECU is showing 60kmh, GPS is also 60; but when ECU is 97 kmh, GPS is already 99.9 kmh! <_<

:o BUT!!! :o

3. When the needle of the speedometer on the dash is just right to the 100kmh mark (as said by RiceRacing that is the "real" 100kph). The OBD/ECU speed is only 92kmh and GPS is 93kmh!!!!!!!

Conclusion:

Kluger Odometer analysis: ECU speed is very close and possibly slightly lower than real GPS recorded speed. And the error shown is mainly on the needle on the dash.

Is there any solution? Yes you can put a OBD display gauge in the car and use the speedo on it, but you have to realise they are very accurate and you have no margin for error (in terms of speeding)!

BTW: This is my try hard run for 0-100. Sometimes the OBD or the GPS lacks and create an error in timing, but looking at the graph, I am actually doing a 7.92 sec !

wow taka thanks for providing this great info. it is really good to know that the speed provided by the OBD port is slow to update and producing those step functions. perhaps when the vehicle speed is decreasing the speed it provides is above, rather than below, the real speed.

i am curious about your OBD reader. i have mainly seen people mention scanguageII and dashhawk. i wonder if you have any thoughts on how your reader compares?

i'd really like to understand the relationship between real vehicle speed (measured by GPS) and the speedo.

i don't have an OBD reader because i have been holding off, waiting to try to get one a bit cheaper. the scanguageII seems to be about $190, the dashhawk is more like $400. but anyway... the experiment i'd like to do is to:

drive along at 20kph according to the speedo and see what the GPS says

drive along at 30kph according to the speedo and see what the GPS says

etc... in 10kph steps up to something not too high unless one is on a track or something!

that way we can get an idea of exactly how the speedo has been "fiddled" in our vehicles: are contant speeds added (with different amounts added in different speed ranges) or is it proportional to speed?

[Taka]

Questions for Taka:

Is the ODB Key a bluetooth device? How much and where from?

How do you do the neat screen images from your phone?

OBD key is a bluetooth device, $180 delivered from obdkey.com

I took the picture with a program called Screenshot

http://www.antonypranata.com/screenshot

Free program!

i am curious about your OBD reader. i have mainly seen people mention scanguageII and dashhawk. i wonder if you have any thoughts on how your reader compares?

i'd really like to understand the relationship between real vehicle speed (measured by GPS) and the speedo.

i don't have an OBD reader because i have been holding off, waiting to try to get one a bit cheaper. the scanguageII seems to be about $190, the dashhawk is more like $400. but anyway... the experiment i'd like to do is to:

drive along at 20kph according to the speedo and see what the GPS says

drive along at 30kph according to the speedo and see what the GPS says

etc... in 10kph steps up to something not too high unless one is on a track or something!

that way we can get an idea of exactly how the speedo has been "fiddled" in our vehicles: are contant speeds added (with different amounts added in different speed ranges) or is it proportional to speed?

OBD reader is a bluetooth device. SO IT IS WIRELESS and draw the power from the OBD port. There is program for Windows CE smart phone and Palm or Windows CE PDAs. So you can use it as a gauge. But it does not come with a display unit. It is faster than CarBus I was told because it is capable of 4Hz update. 4Hz means 4 signals per second. So if you are requiring information from 4 parameters (speed, rpm, intake temp and ambient temp e.g.), you only have one update per second. Certainly it is not as fast as solid wire!

Speedometer calculate the speed from the drive chain, mostly at the differential level so that it will not be affect by gearing. But if the wheel is changed size (e.g. to different proflie, hence wheel circumference)... the speedo will be affected.

GPS is calculated from several coordinates divided by the time travelled. In a way it will be more accurate since most GPS receiver now log on to 6-8 satellites to get the coordinate. The chances of getting accurate result is very high. Most people trust the GPS speed rather than speedo...

The fact that Kluger's dash speed is different from the OBD/ECU speed is that perplex me... :( . Some cars like to display digital speedo and they are directly from the ECU ... even the Echo/Yaris has one!

Nerdygirl, if you want what good speed it is from the GPS, borrow your friends hand held navigator! Most of them show speed and you can use that to mark out what you want to do above! ;) You don't need a OBD gauge... if you really really want one. Check this out http://www.plxdevices.com/

[Taka]

Some one asked me how and where to put the OBD in. It is under the steering wheel column

[PaulG]

Another question: What is the OBD-II protocol that Toyota uses?

[Taka]

Another question: What is the OBD-II protocol that Toyota uses?

Sorry for late reply

It is CAN 11/500 protocol.

Edited July 22, 2008 by Taka

[PaulG]

Thanks Taka.

Love the new Avatar B)

Edited July 22, 2008 by PaulG

[RICE RACING]

Nice work mate ;)

[Taka]

Just a note:

Beware anyone buying the BT Q-818 gps receiver. It is not capable of 5Hz update times, only 1Hz. I found this out the hard way, by jumping in and buying one first before doing the research into it's real capabilities. The BT Q-818 EXTREME is the one you want.

Paul, I don't know how electronically minded you are but you can tweak the unit to do 5Hz.

http://www.gpspassion.com/FORUMSEN/topic.a...amp;whichpage=1

Personally, I don't understand a single thing they talked about. :D

[PaulG]

Thanks Taka, but I already sent that one back. Got another (cheaper) that is 5Hz. Uses the same MTK chipset I believe so all those software applications work just fine. Now just hunting around for a cheap OBD tool!

[Taka]

Which 5Hz did you get?

Make sure you get the OBD key then. It is better than any product out there and I am talking to the developer constantly and there is no other unit better at this stage. Buy it directly from their site, rather than ebay.

Edited August 7, 2008 by Taka

[spolyhronidis]

there's a speed alert in one of the streets in our estate, so i thought i'd test out the speedo. at a indicated speed of 60kph, the sign indicated i was travelling at 55kph.

[Taka]

Just a note:

Beware anyone buying the BT Q-818 gps receiver. It is not capable of 5Hz update times, only 1Hz. I found this out the hard way, by jumping in and buying one first before doing the research into it's real capabilities. The BT Q-818 EXTREME is the one you want.

Paul, I don't know how electronically minded you are but you can tweak the unit to do 5Hz.

http://www.gpspassion.com/FORUMSEN/topic.a...amp;whichpage=1

Personally, I don't understand a single thing they talked about. :D

PaulG, you could have easily kept your Qstarz and need not to return it. You can do any tweak according to the above link. All you need is the GPSview program from that link and change it.

I will prepare what I have done in my blog later. hehe.

[PaulG]

Just a note:

Beware anyone buying the BT Q-818 gps receiver. It is not capable of 5Hz update times, only 1Hz. I found this out the hard way, by jumping in and buying one first before doing the research into it's real capabilities. The BT Q-818 EXTREME is the one you want.

Paul, I don't know how electronically minded you are but you can tweak the unit to do 5Hz.

http://www.gpspassion.com/FORUMSEN/topic.a...amp;whichpage=1

Personally, I don't understand a single thing they talked about. :D

PaulG, you could have easily kept your Qstarz and need not to return it. You can do any tweak according to the above link. All you need is the GPSview program from that link and change it.

I will prepare what I have done in my blog later. hehe.

Hi Taka,

I did try that software and tweak, but it doesn't work. I got information straight from freindly manufacturers saying that model was not capable of being tweaked.

Got my money back without hassle from eBay vendor. Have since bought another unit called a BluCard BT which works very well at 5Hz.

Edited September 7, 2008 by PaulG

[Taka]

Strange, coz the link above uses the exact Qstarz BT-818 non-extreme model.

[spally]

I have access to Survey grade GPS. It runs RTK corrections (30mm positional accuracy) and updates at 20hz. At 100 kph on the GPS, my spedo reads 97kph. Also be aware that "GPS" distance (known as Ellipsoidal Distance") is different to "Ground" distance. The difference depends on your Latitude and Elevation.

[spally]

I have access to Survey grade GPS. It runs RTK corrections (30mm positional accuracy) and updates at 20hz. At 100 kph on the GPS, my spedo reads 97kph. Also be aware that "GPS" distance (known as Ellipsoidal Distance") is different to "Ground" distance. The difference depends on your Latitude and Elevation.

Make that 100 on the speedo and 97 on the gps. ;)

[Kruzenvax]

Well, 3 km/h at 100 is not that bad. When I use my "normal" GPS, e.g. TomTom the difference is more notable.

[Taka]

I have access to Survey grade GPS. It runs RTK corrections (30mm positional accuracy) and updates at 20hz. At 100 kph on the GPS, my spedo reads 97kph. Also be aware that "GPS" distance (known as Ellipsoidal Distance") is different to "Ground" distance. The difference depends on your Latitude and Elevation.

Make that 100 on the speedo and 97 on the gps. ;)

That is not bad at all. I mean the speedo. What sort of GPS is this? Is that bluetooth or via serial bus?

[spally]

I have access to Survey grade GPS. It runs RTK corrections (30mm positional accuracy) and updates at 20hz. At 100 kph on the GPS, my spedo reads 97kph. Also be aware that "GPS" distance (known as Ellipsoidal Distance") is different to "Ground" distance. The difference depends on your Latitude and Elevation.

Make that 100 on the speedo and 97 on the gps. ;)

That is not bad at all. I mean the speedo. What sort of GPS is this? Is that bluetooth or via serial bus?

It is a $35,000 Leica GPS Sensor. It uses both United States' GPS satellites andthe Russian GLONASS sats. It augments its position from fixed base stations via corrections sent over the internet. I am a Naitonal Tech Support person for the survey equipment we sell and I was out looking at satellite coverage on the Gateway Arerial upgrade here in Brizzy. To get over 5hz, you have to go away from bluetooth as BT only supports serial data at a maximum of 5hz.

If you are really serious about getting accurate GPS positions at a high updatae rate, let me know.

[leasaunce]

hmm if your speedo says 94 and in real life you're doing 100kph, thats no good.... would be better if it has been the other way around. I know with my VW, its the other way around....which is a good thing i guess.

[spally]

hmm if your speedo says 94 and in real life you're doing 100kph, thats no good.... would be better if it has been the other way around. I know with my VW, its the other way around....which is a good thing i guess.

leasaunce,

You are right. Is is better the other way. I did correct my first posting to reflect thet the speedo was higher than the GPS. What can I say, It was late at night when I entered my post :)

[Taka]

It is a $35,000 Leica GPS Sensor. It uses both United States' GPS satellites andthe Russian GLONASS sats. It augments its position from fixed base stations via corrections sent over the internet. I am a Naitonal Tech Support person for the survey equipment we sell and I was out looking at satellite coverage on the Gateway Arerial upgrade here in Brizzy. To get over 5hz, you have to go away from bluetooth as BT only supports serial data at a maximum of 5hz.

If you are really serious about getting accurate GPS positions at a high updatae rate, let me know.

Good info about the BT GPS. If 5hz is the limit we have to wait for the next gen bluetooth then. I only want that for autosports purpose. I know about 100Hz VBox that RiceRacer has but I am cheap, plus our races are normally timed anyway.