Just a quick query for those that know, on reading about slow vans ,how do you get you speedo checked.
When I have my very old GPS running there is a difference between it and my speedo.
Which is right? I would hate to think that I was travelling at 90k when I was really travelling a lot slower.
Just got a uhf cb to talk to every one, that may help.
bill12 said
12:31 AM Jan 27, 2012
Think yourself lucky-My speedo and the gps are 15% different.eg. when I am doing 115kph, I am really doing 100kph.The speedo is rather inacurate, to say the least, but at least, if I use the speedo, I am going slower.Bill
Peterpan said
12:45 AM Jan 27, 2012
Thanks Bill,
I think mine is about the same 10 - 15% out according to the GPS. Which one is right?
'Cant tell by the truckies 'cos they always go faster than me.
(I used to be one also.) I don't like holding the traffic up.
Is there a way of checking the GPS?
Kevin
snapper49 said
01:19 AM Jan 27, 2012
Gps is accurate Most speedos on vechicles show around 5kph more than you are actually doing
jimricho said
01:34 AM Jan 27, 2012
My speedo over-reads by 8%. I have confidence in my GPS when on a straight stretch of road in open country. When travelling I set my Garmin to the screen that prominently displays the speed.
Dougwe said
02:36 AM Jan 27, 2012
Hear in South west Gippsland the local police set up a day on a long straight road and get there hair dryer in action. It is publicised a few weeks in advance and you just turn up. They get you to go to a marked point further down the road and drive at a steady 60kmh and they check your speed at another marked point, you do the same thing at 80kmh, they then let you know what speed you were travelling at. Traffic is diverted during this time. It's a great idea and is annual.
dorian said
02:55 AM Jan 27, 2012
My 1996 Toyota Camry and Aldi GPS display the same speed. I'm curious as to how some speedos can be 15% inaccurate.
Are people using lower profile tyres? Have the vehicle manufacturers installed the wrong speedo gear in the gearbox?
Blue Orchid said
03:31 AM Jan 27, 2012
I did the math on our speedo/GPS differences. If I had taken the optional larger wheels on the ranger, then they would have agreed. Speedos are (usually) set to read correctly with the largest tyres fitted at manufacture.
justcruisin01 said
04:21 AM Jan 27, 2012
My speedo is about 7-8 kls slow when the gps reads 100k, this is due to a different profile tyre.
BUT be careful at around the 50/60kl as the differencs is about half.I have used the same gps in a late model family members car with the factory fitted tyres & there was a 10kl difference at 100kls p/hr, just cant remember which way it was so I rely on gps speed.
JC.
dorian said
04:29 AM Jan 27, 2012
In years past, I know that vehicle manufacturers had several different speedo gears to accommodate optional differential ratios. I would think that should still be the case. Moreover, if you have a digital speedometer, then the ratio could be selected by a jumper or programmed into the speedo or engine computer.
Nicholstones said
06:43 AM Jan 27, 2012
Not sure about our speedo, although I suspect it shows slightly faster than reality, but we know our odometer is about 5% short. That is, when the odometer says we have done 100km we have really done about 105km. We take this into account in our fuel and towing logs, so we know how much we have done in real life.
ozjohn said
05:24 PM Jan 28, 2012
dorian wrote:
My 1996 Toyota Camry and Aldi GPS display the same speed. I'm curious as to how some speedos can be 15% inaccurate.
Are people using lower profile tyres? Have the vehicle manufacturers installed the wrong speedo gear in the gearbox?
It's illegal for a speedometer to be 15% inaccurate when the vehicle is fitted with the correct size tyres displayed on the vehicle's Tyre Placard.
Australian Design Rules allow for a speedometer to read up to 10% higher than the true road speed. This can vary if non original after market tyres are fitted. (Most are beteeen 4 - 6% @ 100kph).
The ADR's are as such so as you souldn't get booked for speeding if you match your speed to speed limit applying for that section of road. If you do match your speed, the car will be travelling slower than the speed limit.
Ozjohn
03_troopy said
12:21 AM Jan 30, 2012
The most simple solution is the next time you see a speed trap setup, drive through at about 120KPH. They'll let you know what speed you were doing. But the GPS is the one that will be accurate over a straight line. Just remember that is only updates about once per second though.
Smokeydk said
03:18 AM Jan 30, 2012
Yep.....at 100kph......my gps says I'm doing 89kph......Just came home from few days away.....out in the middle of nowhere.....my GPS went stupid....said I was speeding .(I was doing 92kpg in 110kph zone)......but it said the zone was 50kph......Drrrrrrrrrr
PeterD said
07:14 AM Jan 30, 2012
If you look up ADR-18 you can see the test speeds for spedos. At each test speed the spedo can not read low. It can read high by (10% ot the true speed + 4 km/h.)
jimricho said
03:27 AM Jan 31, 2012
03_troopy wrote:
The most simple solution is the next time you see a speed trap setup, drive through at about 120KPH. They'll let you know what speed you were doing. But the GPS is the one that will be accurate over a straight line. Just remember that is only updates about once per second though.
Don't forget to smile for the picture!
jimricho said
03:31 AM Jan 31, 2012
While my speedo overreads by 8% my odometer is almost "spot on". This suggests that a speed inaccuracy may not be replicated in a corresponding odometer inaccuracy.
Cupie said
01:49 AM Feb 1, 2012
jimricho wrote:
My speedo over-reads by 8%. I have confidence in my GPS when on a straight stretch of road in open country. When travelling I set my Garmin to the screen that prominently displays the speed.
Agree 100%
But I drive by the speedo ... Just to be on the safe side.
I find that the GPS is a bit wonky on undulating terrain & in heavily forrested bits.
I find the changing elevation read out good when the rig seems to be lugging a bit & visually we don't seem to be climbing.
jimricho said
03:58 PM Feb 2, 2012
Agree with your comments, especially about the limitations of GPS in less than ideal conditions.
Smokeydk said
05:43 PM Feb 2, 2012
My Garmin.....done it to me several time.......all.......been on dead flat straight highways.....miles from any town....
it starts beeping WARNINGS.....that I'm exceeding spead limit.......says I should be doing 50kph in a 100/110kph zone
why do this......and sometimes it says its lost satellite .........dont know how many are up there......LOL
ChiChi1 said
05:59 PM Feb 2, 2012
My garmin has had trouble with losing the satelite in the last couple of days. May the weather has something to do with it.
03_troopy said
10:50 PM Feb 2, 2012
The GPS can lose sat signal in heavy cloud cover or forest canopies etc., some moreso than others. It can be worse when they are partially obscured by the metal body of the vehicle they are mounted in as well. Some people have them mounted right in the corner of the windscreen beside the fore corner pillar which can be a problem also. Having said that, a mate of mine has his working with no problems sitting in the centre console of his sedan at times. The thing is, providing you are getting a good signal and the GPS is actually working properly, it will be the most accurate way to measure your speed in a straight line. Here is some interesting info too.
Delboy said
11:03 PM Feb 2, 2012
Weather and the number of satellites visible to your GPS at one time can definetly effect the accuracy. Heavy cloud and other factors slow down the radio signal your GPS is receiving, which effects the accuracy of the calculations performed as the GPS assumes a constant speed for the radio signal to travel from the satellite to your receiver.
It should be remembered that on many vehicles the speedo is a mechanical device and whilst it maybe upto 10% inaccurate by law the error may/will vary at different speeds.
Find a straight road, drive at 100kph on your GPS and note the speedo reading, this will give you a fairly accurate indication of your speedo's error as a percentage (speedo kph - GPS kph = % inaccurate @ 100 kph).
03_troopy said
07:58 PM Feb 3, 2012
Delboy wrote:
Weather and the number of satellites visible to your GPS at one time can definetly effect the accuracy. Heavy cloud and other factors slow down the radio signal your GPS is receiving, which effects the accuracy of the calculations performed as the GPS assumes a constant speed for the radio signal to travel from the satellite to your receiver. It should be remembered that on many vehicles the speedo is a mechanical device and whilst it maybe upto 10% inaccurate by law the error may/will vary at different speeds. Find a straight road, drive at 100kph on your GPS and note the speedo reading, this will give you a fairly accurate indication of your speedo's error as a percentage (speedo kph - GPS kph = % inaccurate @ 100 kph).
Sorry mate, have to disagree with you there. The radio signal will travel at the same speed, which is pretty much the speed of light, no matter what the weather is like. You may however get signal dropout (low signal level) which will affect your refresh rate. So where your GPS would normally refresh at 1Hz, it might actually drop signal for a few seconds and cause innacuracies in immediate readout, because the unit has no idea where it is at that particular time. But once it has locked onto the satellites again it's back to normal.
Delboy said
09:38 PM Feb 3, 2012
I assume we will disagree on this point but...
I was keeping the explanation simple as detailing all the variables that effect RF propogation through the atmosphere is well beyond the scope of this thread.
You are correct drop outs will be higher and inaccuracy increase as reacquisition on each satellite occurs but the critical timing the GPS system is dependent on will also be effected.
PeterD said
09:12 AM Feb 4, 2012
You don't have to rely upon good satellite signals. You use the GPS/navigator once whilst you have a good flat road and a clear sky. Once you have these conditions you calibrate your speedo. Once you have calibrated your speedo there is no need to use the navigator again to check speeds.
Delboy said
05:33 PM Feb 4, 2012
Agree - the process is a once only task, probably worth doing at 100Kph and again at your normal cruising speed, eg 90Kph.
Peterpan said
05:53 PM Feb 4, 2012
Thanks to every one for your input. Huge amount of knowledge and input and diversity. I will try Peterd and Delboys suggestion and see what the variation is.
Kevin
03_troopy said
08:45 PM Feb 4, 2012
Delboy wrote:
I assume we will disagree on this point but... I was keeping the explanation simple as detailing all the variables that effect RF propogation through the atmosphere is well beyond the scope of this thread. You are correct drop outs will be higher and inaccuracy increase as reacquisition on each satellite occurs but the critical timing the GPS system is dependent on will also be effected.
No worries delboy. As being involved with RF transmission & reception, radio navigation systems, radar and in more recent years GPS nav systems etc in aircraft for close to 40yrs I tend to be a bit pedantic about this topic and leap on minor inaccuracies such as this a bit more than I should. Sometimes it's easier just to let it go I suppose. The point I was trying to make though, was that the signal won't arrive any slower. It just may not be available at times and it may take longer for the GPS unit to be able to give any meaningful information. Correct mounting of the unit can help lessen these effects, by having any metal from the car body shielding the GPS as little as possible. I Hope this helps.
PeterD said
12:10 AM Feb 7, 2012
Calibrate - To ascertain the caliber of, as of a thermometer tube; also, more generally, to determine or rectify the graduation of, as of the various standards or graduated instruments. (from Websters.)
As we can not adjust our speedos all we can do is to check and note the accuracy of them. If you are dealing with accurate measuring instruments like test antennas aircraft compases and thermometers they come supplied with calibration charts.
-- Edited by PeterD on Tuesday 7th of February 2012 12:12:06 AM
Just a quick query for those that know, on reading about slow vans ,how do you get you speedo checked.
When I have my very old GPS running there is a difference between it and my speedo.
Which is right? I would hate to think that I was travelling at 90k when I was really travelling a lot slower.
Just got a uhf cb to talk to every one, that may help.
Thanks Bill,
I think mine is about the same 10 - 15% out according to the GPS. Which one is right?
'Cant tell by the truckies 'cos they always go faster than me.
(I used to be one also.) I don't like holding the traffic up.
Is there a way of checking the GPS?
Kevin
Most speedos on vechicles show around 5kph more than you are actually doing
Hear in South west Gippsland the local police set up a day on a long straight road and get there hair dryer in action. It is publicised a few weeks in advance and you just turn up. They get you to go to a marked point further down the road and drive at a steady 60kmh and they check your speed at another marked point, you do the same thing at 80kmh, they then let you know what speed you were travelling at. Traffic is diverted during this time. It's a great idea and is annual.
Are people using lower profile tyres? Have the vehicle manufacturers installed the wrong speedo gear in the gearbox?
My speedo is about 7-8 kls slow when the gps reads 100k, this is due to a different profile tyre.
BUT be careful at around the 50/60kl as the differencs is about half.I have used the same gps in a late model family members car with the factory fitted tyres & there was a 10kl difference at 100kls p/hr, just cant remember which way it was so I rely on gps speed.
JC.
It's illegal for a speedometer to be 15% inaccurate when the vehicle is fitted with the correct size tyres displayed on the vehicle's Tyre Placard.
Australian Design Rules allow for a speedometer to read up to 10% higher than the true road speed. This can vary if non original after market tyres are fitted. (Most are beteeen 4 - 6% @ 100kph).
The ADR's are as such so as you souldn't get booked for speeding if you match your speed to speed limit applying for that section of road. If you do match your speed, the car will be travelling slower than the speed limit.
Ozjohn
If you look up ADR-18 you can see the test speeds for spedos. At each test speed the spedo can not read low. It can read high by (10% ot the true speed + 4 km/h.)
Don't forget to smile for the picture!
Agree 100%
But I drive by the speedo ... Just to be on the safe side.
I find that the GPS is a bit wonky on undulating terrain & in heavily forrested bits.
I find the changing elevation read out good when the rig seems to be lugging a bit & visually we don't seem to be climbing.
it starts beeping WARNINGS.....that I'm exceeding spead limit.......says I should be doing 50kph in a 100/110kph zone
why do this......and sometimes it says its lost satellite .........dont know how many are up there......LOL
Sorry mate, have to disagree with you there. The radio signal will travel at the same speed, which is pretty much the speed of light, no matter what the weather is like. You may however get signal dropout (low signal level) which will affect your refresh rate. So where your GPS would normally refresh at 1Hz, it might actually drop signal for a few seconds and cause innacuracies in immediate readout, because the unit has no idea where it is at that particular time. But once it has locked onto the satellites again it's back to normal.
Thanks to every one for your input. Huge amount of knowledge and input and diversity. I will try Peterd and Delboys suggestion and see what the variation is.
Kevin
No worries delboy. As being involved with RF transmission & reception, radio navigation systems, radar and in more recent years GPS nav systems etc in aircraft for close to 40yrs I tend to be a bit pedantic about this topic and leap on minor inaccuracies such as this a bit more than I should. Sometimes it's easier just to let it go I suppose. The point I was trying to make though, was that the signal won't arrive any slower. It just may not be available at times and it may take longer for the GPS unit to be able to give any meaningful information. Correct mounting of the unit can help lessen these effects, by having any metal from the car body shielding the GPS as little as possible. I Hope this helps.
Calibrate - To ascertain the caliber of, as of a thermometer tube; also, more generally, to determine or rectify the graduation of, as of the various standards or graduated instruments. (from Websters.)
As we can not adjust our speedos all we can do is to check and note the accuracy of them. If you are dealing with accurate measuring instruments like test antennas aircraft compases and thermometers they come supplied with calibration charts.
-- Edited by PeterD on Tuesday 7th of February 2012 12:12:06 AM