Thread: GPS speed readout

Speed info, as long as you have a good sat fix is much more accurate then your speedo.

But in saying that the law doesn't recognise them in Aus and doubtful over there also. So be warned!

Originally Posted by Xavie

Speed info, as long as you have a good sat fix is much more accurate then your speedo.

But in saying that the law doesn't recognise them in Aus and doubtful over there also. So be warned!

that being said, Police equipment reads your real speed, and so should GPS.

Cheers!

the speedo in every land rover I've had has read over the actual speed, by 5%-10% even with new (full diameter) standard tyres

GPS calculates speed from successive position fixes. Given good satellite geometry and reception, this should result in a speed accuracy within better than 1% for typical speeds, but since it is averaged over a time interval (which is likely to be a user setting), the figure may be inaccurate if speed is changing rapidly, as when accelerating or braking.

This assumes that the Yanks do not switch on selective availability (can happen at any time, but is unlikely) in which case accuracy will drop dramatically, depending on the time interval used.

John

An interesting point on GPS speed re-outs as said in one other reply courts will no longer accept GPS speed re-outs as an accurate testimony when the driver is caught speeding they will also no longer accept speedo in-accuracy either as they say it is now up to the owner/driver of the vehicle to make sure the speedo is accurate, ever had a speedo tested for accuracy by an indenpendant tested it cost an arm & a leg to do so.

So all I can say is don't rely on the GPS to get you out of that speeding ticket it just won't happen.

Oh one other thing if the U.S. goes to full scale war as in WW3 the first thing that will go down is the civil GPS system, count on it.
Dingo1

Interesting that the courts are taking such a hard line when the speedo is not accurate when you leave the show room. The ADR rules allow a 10% margin either way to be acceptable. Then there is tyre wear which a speedo has no way of taking into account.

The USA has stated publicly that they reserve the right to selectively or otherwise limit access to GPS in the event of a national security need. Hence the EU has partnered with that other great democracy China to design an alternative system. Difference between them is the while GPS is provided free to everyone this system will only be available to those who are prepared to pay.

Have a friend who is working on the software side of the EU / China system. He has a track record here as designed the software that cruise missles use to find their way. Only person I know who gets excited about the use of cruise missles as it confirms the software actually works. This same software is what makes your portable sat nav work.

Seems the idea is that the EU will also use the new system to introduce a pay as you go system for driving your car. It will also monitor your speed and send a fine if you break a limit and debit your account. Everyone will have a black box in their car / truck and pay accordingly. Think about the amount of data already in your portable sat nav. You cannot do this with GPS as it has only 2 signals while the new one will have 3.

Also GPS is kept inaccurate to about a metre for most users. When combined with the stated intention of the USA to limit access in the event of a national security need is not really good enough if you need it for military accuracy. This it seems to present the EU and China with soverignity integrity issues which this new system will over come.

There are it seems some major (interesting in my mates terms) software gaps with current technology however with the massive budget they have are confident that the pay as you drive system will be working within 10 years.

Enough - enough waffle for now

To know if a vehicle is breaking the speed limit the system would have to know what the speed limit is for every section of road in the country. Then there are the occasions when GPS is not accurate, such as in the "concrete canyons" of big cities and in thick forests, or when the number and/or alignment of visible satellites is less than ideal.

The idea of Big Brother's eye-in-the-sky seems more like pie-in-the-sky.

Stephen.

Originally Posted by 3toes

The ADR rules allow a 10% margin either way to be acceptable.

Current ADR rules state it may only over-read by up to 10%, it must never under-read, IIRC this came into effect 2003 but someone can probably correct me!

I've found running 245/75 16's on my S1 Discovery, the speedo is now within 1% of what my GPS states.

EDIT: Current rules came into effect 2006 IIRC, ADR prior allowed +/- 10% inaccuracies so vehicles plated before that can under-read true speed, but you'd think is a pretty rare occurrence surely unless other mods have been done such as larger tyres etc...??