| Author |
Message |
Nathan
Guest
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 Posted:
Wed Dec 22, 2004 8:54 pm Post subject:
Re: speed accuracy compared to car |
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Sam Wormley wrote:
| Quote: | John Dekker wrote:
Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
Regards,
John.
Keep in mind that most GPS receivers employ "smoothing filters" and so
instantaneous velocity reading during acceleration is not necessarily
accurate. However at constant velocity (and assuming no obstruction of
signals), the GPS receiver will likely measure velocity to an accuracy
of 0.2 m/s (0.7 kph) 2drms.
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Does HDOP affect speed accuracy ? |
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Nathan
Guest
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| Posted:
Wed Dec 22, 2004 9:21 pm Post subject:
Re: speed accuracy compared to car |
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Mxsmanic wrote:
| Quote: | Tumbleweed writes:
Not quite, its based on the idea the manufacturer wont get sued!
I've heard (but I haven't verified) that in some jurisdictions it's
legally required that speedometers erroneously show a speed greater than
a car's actual speed, which seems like the height of folly (rather like
people who knowingly set their clocks 10 minutes ahead to guarantee that
they are on time for meetings or work).
|
In the 1960's US consumer advocate Ralph Nader criticized the
US auto industry with a book entitled "Unsafe at any Speed".
At that time the makers were allowed a +/- 5% error on their
speedometers. A sampling showed American speedometers were all
pretty well showing 5% more and few if any read 5% less.
The problem Nader saw at the time was that car warranties were based
on mileage. If the speedo read high, then the odometer read high too.
Cars were going off warranty 5% sooner than they should. This was saving
the auto industry millions of dollars in repair claims.
Car rental companies were also making out like bandits. Most had a
mileage charge. It was nice to be getting an extra 5% out of customers
when you had huge fleets of cars rolling on the highways 24/7.
Nader lobbied to get this changed, and the car companies replied
with the argument that it was too difficult to get the error rate
of less than 5%.
A law was eventually passed requiring speedos to be within 2%. The
industry complied, and most cars tested read 2% high rather than 5% high,
but none read 2% low
That was in the 60's.. I'm sure they're far more accurate now, and
perhaps they all do read a toch low... (My vehicle does). But, if they
do, it's for more than just worries about speeding.. |
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Rob
Guest
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| Posted:
Wed Dec 22, 2004 9:41 pm Post subject:
Re: speed accuracy compared to car |
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"John Dekker" <jdekker@xtra.co.nz> wrote in message
news:S_6yd.371$mo2.27870@news.xtra.co.nz...
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph?
Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
Regards,
John.
My observations are that in UK most cars have read in the region of 7 to10% |
higher than the GPS speed. In US every car I've driven has been much
closer to the GPS speed (usually indistinguishable given normal
fluctuations). These seem to tie in with the harder facts re UK laws etc
that others point out. Sounds like NZ is closer to UK in this repect. |
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Sam Wormley
Guest
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| Posted:
Wed Dec 22, 2004 10:20 pm Post subject:
Re: speed accuracy compared to car |
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Nathan wrote:
| Quote: | Sam Wormley wrote:
John Dekker wrote:
Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
Regards,
John.
Keep in mind that most GPS receivers employ "smoothing filters" and so
instantaneous velocity reading during acceleration is not necessarily
accurate. However at constant velocity (and assuming no obstruction of
signals), the GPS receiver will likely measure velocity to an accuracy
of 0.2 m/s (0.7 kph) 2drms.
Does HDOP affect speed accuracy ?
|
Slightly--remember horizontal speed is determined from the Dopple
shift in the signals. |
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Hans-Georg Michna
Guest
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| Posted:
Thu Dec 23, 2004 12:32 am Post subject:
Re: speed accuracy compared to car |
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On Wed, 22 Dec 2004 09:43:08 -0500, David Phillips
<david.phillips@sas.com> wrote:
| Quote: | I'll agree that car speedometers are notoriously bad, but argue with
the accuracy change along with tire wear. There is some, but it's
only about 2%.
|
David,
the difference between different types of tire may be bigger
though. The car's speedometer still must not underread when you
mount a bigger type of tire.
Tire pressure has also been mentioned.
Hans-Georg
--
No mail, please. |
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Juergen Nieveler
Guest
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| Posted:
Thu Dec 23, 2004 12:43 am Post subject:
Re: speed accuracy compared to car |
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Joe Parsons <asad@yankeemedia.n3t> wrote:
| Quote: | It's a pretty safe bet your car speedo is more than a little
optimistic. The GPS speeds are *very* accurate, from my
understanding.
|
Not necessarily optimistic, but rather erring on the side of caution.
If you get a speeding ticket, saying "My speedo showed I was driving
slower than I really was" would be a good defense. The manufacturer
would be responsible in that case - and so they build it so that it
always shows a few percent more speed.
Juergen Nieveler
--
I call things as I see them; If I didn't see them, I make them up! |
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Juergen Nieveler
Guest
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| Posted:
Thu Dec 23, 2004 12:43 am Post subject:
Re: speed accuracy compared to car |
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Nathan <nat.keller@mls.net> wrote:
| Quote: | The problem Nader saw at the time was that car warranties were based
on mileage. If the speedo read high, then the odometer read high too.
Cars were going off warranty 5% sooner than they should. This was
saving the auto industry millions of dollars in repair claims.
|
Ever taken a speedometer apart? The odometer gets driven by a flexible
wire that registers how many turns the wheel makes. The speedometer
however is driven indirectly via a turning magnet working against a
spring, "dragging" the needle. The precision of the speedo depends
mostly on the spring, whereas the odometer only is influenced by the
wheel.
Juergen Nieveler
--
When the pin is pulled, Mr. Grenade is not our friend. |
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Juergen Nieveler
Guest
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| Posted:
Thu Dec 23, 2004 12:43 am Post subject:
Re: speed accuracy compared to car |
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Hans-Georg Michna <hans-georgNoEmailPlease@michna.com> wrote:
| Quote: | It is like that in Germany and probably in many more countries.
The rule is actually that the speedometer must never show a
lower speed than the actual speed.
|
Except for trucks, IIRC. They must get their speedometer calibrated
regularly. That's why you sometimes see trucks driving faster than you
think is allowed - they KNOW how fast they're driving and are exactly
following the limit, whereas your speedometer shows a bit too much
speed.
Juergen Nieveler
--
You're only as old as you feel... the next day |
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Stan Gosnell
Guest
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| Posted:
Thu Dec 23, 2004 6:05 am Post subject:
Re: speed accuracy compared to car |
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David Phillips <david.phillips@sas.com> wrote in
news:d02js09p35lpdipqs849p4s7u3iplplt2s@4ax.com:
| Quote: | My car was brought to my state from a dealer two states away. My
speedometer is exactly correct (within the limits of my ability to
measure, via stopwatch and mileage posts, and with a borrowed GPS)
|
But lower your tire pressure by 5PSI, and wear down your tire tread by
1/16 inch, and see how accurate it is.
--
Regards,
Stan |
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JGS
Guest
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| Posted:
Thu Dec 23, 2004 6:30 am Post subject:
Re: speed accuracy compared to car |
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On Wed, 22 Dec 2004 09:46:11 -0500, David Phillips <david.phillips@sas.com>
wrote:
| Quote: | On Wed, 22 Dec 2004 11:14:03 +0100, Mxsmanic <mxsmanic@hotmail.com
wrote:
Tumbleweed writes:
Not quite, its based on the idea the manufacturer wont get sued!
I've heard (but I haven't verified) that in some jurisdictions it's
legally required that speedometers erroneously show a speed greater than
a car's actual speed, which seems like the height of folly (rather like
people who knowingly set their clocks 10 minutes ahead to guarantee that
they are on time for meetings or work).
I would be surprised to find that is actually correct. While I
obviously haven't looked at the motor vehicle code in every
jurisdiction, I've never seen it in my state's laws. It would be a
logistical nightmare for the manufacturer to have an accurate speedo
in one state or country, and an 'optimistic' speedo in another.
My car was brought to my state from a dealer two states away. My
speedometer is exactly correct (within the limits of my ability to
measure, via stopwatch and mileage posts, and with a borrowed GPS)
|
I have a 2004 Dodge Intrepid with about 10,000 miles on it. I have compared
the speedometer to the speed indicated by my Garmin 60C and discern any
difference (by eyeballing the speedometer) at a constant speed of 65 mph. In
the case of the Dodge, tire wear is not an issue and tire inflation is
correct for the auto.
Dodge does not seem to intentionally calibrate the speedometer to read
faster than actual.
Luck of the draw?
Regards
JGS |
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Peter
Guest
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| Posted:
Thu Dec 23, 2004 6:35 am Post subject:
Re: speed accuracy compared to car |
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Stan Gosnell wrote:
| Quote: | David Phillips <david.phillips@sas.com> wrote in
news:d02js09p35lpdipqs849p4s7u3iplplt2s@4ax.com:
My car was brought to my state from a dealer two states away. My
speedometer is exactly correct (within the limits of my ability to
measure, via stopwatch and mileage posts, and with a borrowed GPS)
But lower your tire pressure by 5PSI, and wear down your tire tread by
1/16 inch, and see how accurate it is.
|
Probably still very accurate. Assuming the tire diameter is
about 24" (radius 12"), then the 1/16" of tread wear would
only affect circumference by half a percent (1/16/12).
The effect of the decreased pressure depends on the tire
construction. Most tires these days are steel-belted radials
where the circumference is essentially fixed by the length of
the belt immediately below the tread regardless of inflation
pressure (given enough pressure to avoid complete collapse of
the tire). I once tried to measure the effect of dropping
pressure by 10 psi (from 30 to 20) and found that it seemed
to only be about 1%, but was hard to measure. So I'd guess
the total effect to only be about 1% or 0.5 mph at 50 mph.
Since many speedometers are hard to read to better than
1 mph anyway this probably wouldn't even be noticed.
BTW, the same pressure test on bias ply tires showed a
much greater dependence. |
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Robert Peirce
Guest
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| Posted:
Thu Dec 23, 2004 7:23 am Post subject:
Re: speed accuracy compared to car |
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|
In article <S_6yd.371$mo2.27870@news.xtra.co.nz>,
"John Dekker" <jdekker@xtra.co.nz> wrote:
| Quote: | Any ideas on what the accuracy of a handheld GPS is, ie 0.1 mph or 1mph? Its
just that I was driving my car and noticed the car was showing about 7%
faster than my Garmin 60cs.
Regards,
John.
|
Easy to check in the US. All interstates have mileposts. If you have
cruise control and a similar situation, just set your speed and time it.
I use my Garmin StreetPilot to set my cruise control. Among my cars,
some match and some are higher. None are lower. My wife's Accord
registers about 78 when she is really doing 73-74. My daughter's
Accord, gong the same speed, is pretty much dead on.
--
Robert B. Peirce, Venetia, PA 724-941-6883
bob AT peirce-family.com [Mac]
rbp AT cooksonpeirce.com [Office] |
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Dale DePriest
Guest
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| Posted:
Thu Dec 23, 2004 8:14 am Post subject:
Re: speed accuracy compared to car |
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True enough. There is no reason to believe the odometer and speedometer
have the same error. Error also varies with speed. Some cars have the
calibration marks right on the speedometer. Many Fords do and you can
see that the calibration allows for reading high and not low. At high
speeds 80mph the discrepency is pretty large. They don't even care above
that.
Dale
Juergen Nieveler wrote:
| Quote: | Nathan <nat.keller@mls.net> wrote:
The problem Nader saw at the time was that car warranties were based
on mileage. If the speedo read high, then the odometer read high too.
Cars were going off warranty 5% sooner than they should. This was
saving the auto industry millions of dollars in repair claims.
Ever taken a speedometer apart? The odometer gets driven by a flexible
wire that registers how many turns the wheel makes. The speedometer
however is driven indirectly via a turning magnet working against a
spring, "dragging" the needle. The precision of the speedo depends
mostly on the spring, whereas the odometer only is influenced by the
wheel.
Juergen Nieveler
|
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs |
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L.D.
Guest
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| Posted:
Thu Dec 23, 2004 9:56 pm Post subject:
Re: speed accuracy compared to car (OT) |
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Peter wrote:
| Quote: | Stan Gosnell wrote:
David Phillips <david.phillips@sas.com> wrote in
news:d02js09p35lpdipqs849p4s7u3iplplt2s@4ax.com:
My car was brought to my state from a dealer two states away. My
speedometer is exactly correct (within the limits of my ability to
measure, via stopwatch and mileage posts, and with a borrowed GPS)
But lower your tire pressure by 5PSI, and wear down your tire tread
by 1/16 inch, and see how accurate it is.
Probably still very accurate. Assuming the tire diameter is
about 24" (radius 12"), then the 1/16" of tread wear would
only affect circumference by half a percent (1/16/12).
The effect of the decreased pressure depends on the tire
construction. Most tires these days are steel-belted radials
where the circumference is essentially fixed by the length of
the belt immediately below the tread regardless of inflation
pressure (given enough pressure to avoid complete collapse of
the tire). I once tried to measure the effect of dropping
pressure by 10 psi (from 30 to 20) and found that it seemed
to only be about 1%, but was hard to measure. So I'd guess
the total effect to only be about 1% or 0.5 mph at 50 mph.
Since many speedometers are hard to read to better than
1 mph anyway this probably wouldn't even be noticed.
BTW, the same pressure test on bias ply tires showed a
much greater dependence.
Where in the world did you find the bias ply tires for the test? I've |
been looking for bias ply tires for several years and only ones I've
found are for trailer use only. |
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Peter
Guest
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| Posted:
Thu Dec 23, 2004 10:01 pm Post subject:
Re: speed accuracy compared to car (OT) |
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|
L.D. wrote:
| Quote: | Peter wrote:
Stan Gosnell wrote:
My car was brought to my state from a dealer two states away. My
speedometer is exactly correct (within the limits of my ability to
measure, via stopwatch and mileage posts, and with a borrowed GPS)
But lower your tire pressure by 5PSI, and wear down your tire tread
by 1/16 inch, and see how accurate it is.
Probably still very accurate. Assuming the tire diameter is
about 24" (radius 12"), then the 1/16" of tread wear would
only affect circumference by half a percent (1/16/12).
The effect of the decreased pressure depends on the tire
construction. Most tires these days are steel-belted radials
where the circumference is essentially fixed by the length of
the belt immediately below the tread regardless of inflation
pressure (given enough pressure to avoid complete collapse of
the tire). I once tried to measure the effect of dropping
pressure by 10 psi (from 30 to 20) and found that it seemed
to only be about 1%, but was hard to measure. So I'd guess
the total effect to only be about 1% or 0.5 mph at 50 mph.
Since many speedometers are hard to read to better than
1 mph anyway this probably wouldn't even be noticed.
BTW, the same pressure test on bias ply tires showed a
much greater dependence.
Where in the world did you find the bias ply tires for the test? I've
been looking for bias ply tires for several years and only ones I've
found are for trailer use only.
|
The test was done quite a while ago. Bias ply tires used to
be quite common. |
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