Satnav - v- speedo readings..............?
15 July 2005, 07:04 PM
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Satnav - v- speedo readings..............?
Hi all, just a quickie, what does your satnav read when speedo shows 80mph?
Mine shows 73 mph..................about right?
Si.
Mine shows 73 mph..................about right?
Si.
15 July 2005, 07:14 PM
#3
It depends on the make of car and the level of over-read in the speedo. It also depends on what speed you are doing and whether the satnav updates it speed reading in real time or, say, every 2 seconds.
For me my GPS updates every 2 seconds. If I hold the car at 30mph steady for longer than that (not easy to do) to validate against the GPS unit my speedo over-reads by 2mph, ie GPS unit says 28mph. The assumption made here is that the GPS readings are accurate.
If I hold a steady 70mph then my speedo over-reads by 3mph, ie GPS says 67mph.
Hope this helps.
For me my GPS updates every 2 seconds. If I hold the car at 30mph steady for longer than that (not easy to do) to validate against the GPS unit my speedo over-reads by 2mph, ie GPS unit says 28mph. The assumption made here is that the GPS readings are accurate.
If I hold a steady 70mph then my speedo over-reads by 3mph, ie GPS says 67mph.
Hope this helps.
15 July 2005, 07:31 PM
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There's all sorts of tolerance stack-ups in the drivetrain system that can make the speedo "inaccurate" so that's why speedos are allowed to read up to 10% over but not under-read.
For example, as your tyres wear down, the speedo reads faster as you're effectively altering the gearing ratio...
gps is the most accurate signa you'r going to get in normal money terms as it's just calculating time and distance from the satellite to work out speed - no car inacuracies involved
As WB mentioned, gps only updates every second or so which means you need to hold a constant speed to compare the car and gps - there's no point trying to do 0-60 tests with sat-nav
My bugeye speedo was 10% out at all speeds but my STi is more accurate but doesn't seem to be as linear so I'll have to have an experimenting session
For example, as your tyres wear down, the speedo reads faster as you're effectively altering the gearing ratio...
gps is the most accurate signa you'r going to get in normal money terms as it's just calculating time and distance from the satellite to work out speed - no car inacuracies involved
As WB mentioned, gps only updates every second or so which means you need to hold a constant speed to compare the car and gps - there's no point trying to do 0-60 tests with sat-nav
My bugeye speedo was 10% out at all speeds but my STi is more accurate but doesn't seem to be as linear so I'll have to have an experimenting session
15 July 2005, 07:52 PM
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Originally Posted by Apple
gps is the most accurate signa you'r going to get in normal money terms as it's just calculating time and distance from the satellite to work out speed - no car inacuracies involved
We had this discussion here some time back and there was one person who posted convinced that his GPS was doing a distance/time calculation. After some discussion, and to considerable amusement, it turned out that he was flying Hurcules transport aircraft and their GPS does indeed work out velocity in this manner. However, chances are any unit you will have access to will use doppler.
In the past, when SA was turned on, there was an easy way to prove this as domestic handheld units could give you a good speed reading before you had walked a distance equal to the average error introduced by SA. It is amazing the workings of a wheelbarrow, as they say.
15 July 2005, 08:15 PM
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At the risk of re-covering old ground; it'd be a mistake to presume that the GPS kit to which we commonly have access is as completely accurate.
Of course, as this is ScoobyNet there'll be someone along in a minute who will argue with me, whilst plainly knowing f*ck-all.....
Of course, as this is ScoobyNet there'll be someone along in a minute who will argue with me, whilst plainly knowing f*ck-all.....
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16 July 2005, 08:42 PM
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Afraid I can't find it, did try a search etc. but no joy. I'm pretty sure it was on here but I suppose there is a chance that it was somewhere else or even an email exchange. I have some vague recollection of us resorting to calculus at some point in the discussion, but maybe that was another discussion.
Do you think using GPS, or discussing it, rots your brain and causes you to lose your memory?
Do you think using GPS, or discussing it, rots your brain and causes you to lose your memory?
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