As for the Police, the law states that the speed limit is a LIMIT, not the recommended driving speed. Its the maximum speed you can travel. If you try to travel exactly at that speed and get it wrong, that's a risk you take and that's the view the law takes. According to my experience, if you travel on or just below the speed limit using your Speedo as a reference, you will never get a speeding ticket. That's assuming your vehicle's Speedo and tyres are within specifications.
There is a software modification that can be done to the BA Ford Falcon range that displays the 'electronic' speed in digital form at the bottom of the Speedo. Its used for Police vehicles and the additional speed display runs directly from the speed pulse sensor. That reading is usually within 1% of the GPS measured signal and indicates that the real speed of the BA range is around 3% lower than the standard mechanical Speedo dial indicates.
BTW, when measuring the GPS speed, all consumer GPS units have a filter that averages the speed over time. You need to travel in a straight line at a constant speed for a few seconds before the speed reading stabilises enough to be considered accurate (to around 0.05 meters per second or about 0.18 kph).
Ian
----- Original Message ----- From: "Allan Armistead" <[EMAIL PROTECTED]>
To: "Discussion of issues relating to Soaring in Australia." <aus-soaring@lists.internode.on.net>
Sent: Tuesday, May 17, 2005 2:52 PM
Subject: [Aus-soaring] Off topic - speedo accuracy
I'd like to ask for the indulgence of the group to go a bit off topic to gliding.
Those of you who are also members of NRMA might have seen a letter (and editorial comment) in their last edition about speedo accuracy.
As it relates to an area of interest of mine (roads/road safety) I'm intending to respond on the matter and thought it would be useful to do a straw poll of people who have the ability to "calibrate" their speedos, to get a reasonable idea of the order of accuracy across a range of makes and models. I know both my cars are optimistic by around 6-8km/h at highway speeds.
So who better to ask than people who have access to speed measuring equipment (GPS) and drive a range of vehicles on the highways and byways (glider pilots).
If anyone is interested in giving me some input into this, I would ask you to do the following - safely, on a quiet road, at legal speeds and with a passenger to read the GPS;
. drive at steady speeds of 80, 100 and (if possible/legal) 110km/h as indicated on your vehicle speedo,
. note the GPS reading at each of these indicated speeds
. tabulate the results, together with the make, model and year model of the
vehicle,
. email them direct to me at [EMAIL PROTECTED] (please do not clog up the newsgroup with the results)
Apart from my specific interest in the matter, it is useful information to
have for your own purposes. Australian Design Rules require a speedo
accuracy of +-10%. My experience/observation is that speedos are generally a
bit optimistic, but it is quite possible that if you are reading 100km/h you
are actually doing somewhere between 90 and 110 - which could be important
to know if you are in Victoria with it's 3km/h tolerance level.
Allan Armistead ph (02) 6249 6470, fax (02) 6249 6555, mobile 0413 013 911 PO Box 908, Dickson ACT 2602, Australia
"When once you have tasted flight, you will always walk with your eyes turned skyward, for there you have been and there you always will be." Leonardo da Vinci, 1452-1519
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