On 12 Feb, 13:44, Roland Perry wrote:
In message
, at
05:02:55 on Tue, 12 Feb 2008, Mizter T remarked:

I've never really used a sat-nav system

Do they display a GPS speed? I use a rather older handheld GPS system
(that doesn't have maps).


I think some do.


to compare true speeds with indicated speeds, so I've never really been
aware of the difference between the 'true' speed I've been travelling
at as opposed to the 'indicated' speed.

My current car indicates 60mph at a true 56mph. And similarly pro-rata
at higher speeds.


Interesting, that's a significant enough difference.


I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

They are legally required to. Any error *has* to mean the speedo is
over-reading.


I didn't realise it was a legal requirement, but of course it's
perfectly logical that it is. To be honest I hadn't ever spent much
time thinking about it!

In message , at 14:10:50 on Tue,
12 Feb 2008, John Rowland
remarked:
I've never really used a sat-nav system

Do they display a GPS speed?

TomTom does. It gives lat long as well.

It doesn't give height, though. Is it hard for GPS to be used to determine
height above sea level?

Height is much less accurate than position. It's also of little use to a
driver trying to find his way (rather than, say, a hiker on foot with a
map and contour lines, or an aircraft pilot).
--
Roland Perry

At 13:44:35 on Tue, 12 Feb 2008 Roland Perry opined:-

My current car indicates 60mph at a true 56mph. And similarly pro-rata
at higher speeds.

I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

They are legally required to. Any error *has* to mean the speedo is
over-reading.

ISTR that the Construction and Use Regulations require speedos to be
accurate to +/-10%. I don't think you can rely on yours over-reading.
--
Thoss

In message , at 18:20:31 on Tue, 12 Feb
2008, thoss remarked:
My current car indicates 60mph at a true 56mph. And similarly pro-rata
at higher speeds.

I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

They are legally required to. Any error *has* to mean the speedo is
over-reading.

ISTR that the Construction and Use Regulations require speedos to be
accurate to +/-10%. I don't think you can rely on yours over-reading.

Absolutely no margin allowed for under-reading. Sources differ slightly
with regard to over-reading, but the limit seems to be +10% + 4kph (from
Hansard in 2001).

As there are apparently anything up to seven amendments *per year* to
the Construction and Use Regulations, I'll leave it to someone else to
find the exact cite.
--
Roland Perry

On Tue, 12 Feb 2008 05:02:55 -0800 (PST), Mizter T
wrote:

I've never really used a sat-nav system to compare true speeds with
indicated speeds, so I've never really been aware of the difference
between the 'true' speed I've been travelling at as opposed to the
'indicated' speed.

Be careful if you do use a sat-nav. If you're going up or downhill,
it will read slower because (so far as the satellites can see) you've
slowed down because your speed has a vertical component as well.

I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

Yes, as it's illegal for one to read even 1mph under, but may read up
to 5% (I think) over. Thus, they all read over.

Neil

--
Neil Williams
Put my first name before the at to reply.

"thoss" wrote in message

At 13:44:35 on Tue, 12 Feb 2008 Roland Perry opined:-

My current car indicates 60mph at a true 56mph. And similarly
pro-rata at higher speeds.

I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

They are legally required to. Any error *has* to mean the speedo is
over-reading.

ISTR that the Construction and Use Regulations require speedos to be
accurate to +/-10%. I don't think you can rely on yours over-reading.

I have a hand-held GPS that shows my car's speedo over-reads by about 6%
or 7%. One oddity is that my cruise control allows me to set an exact
speed (digitally), which then doesn't agree with analogue speedo (even
though I assume they are both driven off the same digital source,
probably in the gear-box). My GPS shows that the analogue speedo is more
accurate than the cruise control. So, if I want to drive at exactly
70mph, I would set the cruise control at 75 or 76 mph.

One reason speedos have to over-read is that even the most precise
speedo driven from the drive train will not be exactly right -- it
depends on your tyre pressures and air temperature, as well as tyre
characteristics. If you have been driving at speed for a while, the
tyres will warm up, increasing the pressure in the tyres, and hence
their rolling radius. So, with the exact same speedo reading (and cruise
control setting), the car will actually travel a bit faster.

My car is available with 18", 19" and 20" wheels. All have tyres with
the same nominal external diameter, but the higher profile tyres fitted
to the smaller diameter rims will be less stiff than the ultra-low
profile tyres fitted to the 20" rims. The latter will therefore have a
slightly larger rolling radius, so the car will be travelling a bit
faster with the same speedo reading. The rolling radius will reduce
slightly if the car is heavily loaded.

This is all in addition to any manufacturing tolerances in the speedo
itself. That's why manufacturers typically aim at a +5% reading, so the
actual readings will be in the 0 to +10% error range.

On 12 Feb, 14:10, "John Rowland"
wrote:
It doesn't give height, though. Is it hard for GPS to be used to determine
height above sea level?

A GPS receiver works by narrowing down your position in three-
dimensional space, so figuring out your altitude and figuring out your
location are inseparable.

U

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

On Tue, 12 Feb 2008, Neil Williams wrote:

On Tue, 12 Feb 2008 05:02:55 -0800 (PST), Mizter T
wrote:

I've never really used a sat-nav system to compare true speeds with
indicated speeds, so I've never really been aware of the difference
between the 'true' speed I've been travelling at as opposed to the
'indicated' speed.

Be careful if you do use a sat-nav. If you're going up or downhill,
it will read slower because (so far as the satellites can see) you've
slowed down because your speed has a vertical component as well.

Unless your box is using the height it's inseparably calculated in
determining your speed!

If it was, it could tell you the gradient you're climbing, too.

tom

--
No hay banda

Mr Thant wrote:
On 12 Feb, 14:10, "John Rowland"
wrote:
It doesn't give height, though. Is it hard for GPS to be used to
determine height above sea level?

A GPS receiver works by narrowing down your position in three-
dimensional space, so figuring out your altitude and figuring out your
location are inseparable.

If only three satellites are visible, the locus of possible locations is a
straight line in space which intersects the earth's surface at two points.
Knowing which satellites are visible enables you to eliminate one of these
points, but it doesn't give you the height. Even if quite a few satellites
are visible which were all roughly in a plane, there would be low precision
on the height, and if all the satellites were exactly in a plane, there
would be no information to calculate the height.

At 23:19:40 on Tue, 12 Feb 2008 Tom Anderson opined:-

On Tue, 12 Feb 2008, Neil Williams wrote:

On Tue, 12 Feb 2008 05:02:55 -0800 (PST), Mizter T
wrote:

I've never really used a sat-nav system to compare true speeds with
indicated speeds, so I've never really been aware of the difference
between the 'true' speed I've been travelling at as opposed to the
'indicated' speed.

Be careful if you do use a sat-nav. If you're going up or downhill,
it will read slower because (so far as the satellites can see) you've
slowed down because your speed has a vertical component as well.

Unless your box is using the height it's inseparably calculated in
determining your speed!

If it was, it could tell you the gradient you're climbing, too.

Since we're here talking sat-navs and speed limits, and since sat-navs
have maps, measure speeds and know about speed limits, are there any
which issue a warning when the local limit is being exceeded?
--
Thoss