Speedo Check.

Peterpan · Guru

Speedo Check.

bill12 · Guru

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 · Guru

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 · Senior Member

Gps is accurate
Most speedos on vechicles show around 5kph more than you are actually doing

jimricho · Guru

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 · Chief one feather

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 · Guru

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 · Senior Member

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 · Guru

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 · Guru

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 · Guru

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 · Guru

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 · Guru

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 · Guru

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 · Guru

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 · Guru

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 · Guru

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 · Guru

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 · Guru

Agree with your comments, especially about the limitations of GPS in less than ideal conditions.

Smokeydk · Guru

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 · Guru

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 · Guru

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 · Member

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 · Guru

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 · Member

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 · Guru

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 · Member

Agree - the process is a once only task, probably worth doing at 100Kph and again at your normal cruising speed, eg 90Kph.

Peterpan · Guru

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 · Guru

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 · Guru

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