| Author |
Message |
Jeff Strickland
Guest
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 Posted:
Sun Dec 04, 2005 12:54 am Post subject:
Re: speedometer not properly calibrated |
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Why would you even consider asking a mechanic to check this? Get out on the
freeway and drive a measured mile at a fixed speed and see how long it
takes. Then divide 3600 by the time (in seconds) that it took to travel the
mile, the result should be your set speed within a couple of mph. The
displayed speed, 70 for example, will be about 2 mph faster than the actual
speed.
TIMES NEEDED TO GO 1 MILE
60 mph, 60 seconds
70 mph, 51.42 seconds
80 mph, 45 seconds
You can divide 3600 by any MPH to arrive at the time in seconds that it
takes to travel 1 mile, then you can go out and set the Cruise, or carefully
use the gas pedal to maintain a constant speed, and verify the time. The
Interstate Highways all have mile markers along the right shoulder that are
spaced within a few feet of precisely 1-mile intervals. There will be a
slight delay in your timing to start and stop your stop watch, but you can
easily confirm if your speedo is accurate or not. Obviously, if you got a
speeding ticket doing 50 in a 35, you will find that your speedo is much too
accurate to let this happen. If you car has oversized tires on it --
generally this is something that only affects trucks -- then you _might_
have a speedo that reads 35 when you are really doing 50, but all passenger
cars in America -- and the rest of the world for that matter -- have a
speedo that reads something in the range of 3 mph faster than your actual
speed. The purpose of this is to keep you from getting speeding tickets.
"sh-aly" <wqadeer@stanford.edu> wrote in message
news:dmhbac$g7g$1@news.Stanford.EDU...
| Quote: | How can you check if the speedometer is not properly calibrated and if I
ask a mechanic to do it how much should it cost?
Thanks,
W
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Jeff Strickland
Guest
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| Posted:
Sun Dec 04, 2005 12:55 am Post subject:
Re: speedometer not properly calibrated |
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"Gord Beaman" <gord@islandtelecom.com> wrote in message
news:jdk8r1dvp0fho2m04ejsnskugaaa827uj8@4ax.com...
| Quote: | "B. Peg" <bent_peg@worldnet.att.net> wrote:
How can you check if the speedometer is not properly calibrated and if I
ask a mechanic to do it how much should it cost?
If you belong to the Auto Club, they have a roving speedometer check
vehicle. They post its location in their magazine, which is usually at
one
of their local offices. I believe it is a free check.
The GPS idea is also good.
B~
But how accurate is the GPS?...I have a Magellan Meridan
Gold...not very expensive (or is expense a factor?)
--
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GPS is close enough to check a speedo. |
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Gord Beaman
Guest
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| Posted:
Sun Dec 04, 2005 2:04 am Post subject:
Re: speedometer not properly calibrated |
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"Jeff Strickland" <crwlr@yahoo.com> wrote:
| Quote: |
"Gord Beaman" <gord@islandtelecom.com> wrote in message
news:jdk8r1dvp0fho2m04ejsnskugaaa827uj8@4ax.com...
"B. Peg" <bent_peg@worldnet.att.net> wrote:
How can you check if the speedometer is not properly calibrated and if I
ask a mechanic to do it how much should it cost?
If you belong to the Auto Club, they have a roving speedometer check
vehicle. They post its location in their magazine, which is usually at
one
of their local offices. I believe it is a free check.
The GPS idea is also good.
B~
But how accurate is the GPS?...I have a Magellan Meridan
Gold...not very expensive (or is expense a factor?)
--
GPS is close enough to check a speedo.
How do you know? |
--
-Gord.
(use gordon in email)
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Jeff Strickland
Guest
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| Posted:
Sun Dec 04, 2005 9:18 am Post subject:
Re: speedometer not properly calibrated |
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"Gord Beaman" <gord@islandtelecom.com> wrote in message
news:gi1jr15gs5fchg0qopt7do1a89fsp8o5n4@4ax.com...
| Quote: | "Jeff Strickland" <crwlr@yahoo.com> wrote:
"Gord Beaman" <gord@islandtelecom.com> wrote in message
news:jdk8r1dvp0fho2m04ejsnskugaaa827uj8@4ax.com...
"B. Peg" <bent_peg@worldnet.att.net> wrote:
How can you check if the speedometer is not properly calibrated and if
I
ask a mechanic to do it how much should it cost?
If you belong to the Auto Club, they have a roving speedometer check
vehicle. They post its location in their magazine, which is usually at
one
of their local offices. I believe it is a free check.
The GPS idea is also good.
B~
But how accurate is the GPS?...I have a Magellan Meridan
Gold...not very expensive (or is expense a factor?)
--
GPS is close enough to check a speedo.
How do you know?
|
Because I sold GPS systems, and have a pretty good idea of how the system
actually works. It's all based on known mathematical formulas and a minimum
of 4 (four) satellites. It's easy to guage the speed. The location is an
easy task as well, but the early systems were not allowed to be accurate on
the location, but the rules have changed since then. Now, the GPS systems
are accurate to within 3 meters (about 10 feet), but this degree of accuracy
has nothing to do with the speed. |
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Bonehenge
Guest
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| Posted:
Sun Dec 04, 2005 6:59 pm Post subject:
Re: speedometer not properly calibrated |
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On Sat, 3 Dec 2005 23:45:38 -0800, "Jeff Strickland" <crwlr@yahoo.com>
wrote:
| Quote: | Because I sold GPS systems, and have a pretty good idea of how the system
actually works. It's all based on known mathematical formulas and a minimum
of 4 (four) satellites. It's easy to guage the speed.
|
The main issue with speed was sampling rate, or the number of times in
a time period that the unit calculated a position.
My old Garmin GPS II and III (early and mid 90's vintage) would lag
the speedo quite a bit during spirited acceleration and deceleration.
The eTrex Vista and aviation GPSMAP 196 I have are better, while my
GPSMAP 60CS has an almost non-existent lag. The lag is evident as the
GPS displays below the speedo while accelerating, and above the
speedo's indicated speed while decelerating.
Slamming on the brakes with the older units might have you at a dead
stop with the GPS reading 4-5 MPH for a second or two, or maybe a 5
MPH reading, then a 2, then a 0. Accelerating down a runway in a jet
might see it lag behind 10-20 MPH.
These examples were all with selective availability turned off.
At a relatively constant speed, I've never seen a GPS that wasn't dead
on compared to a timed distance. |
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Scott in Florida
Guest
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| Posted:
Sun Dec 04, 2005 8:09 pm Post subject:
Re: speedometer not properly calibrated |
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On Sun, 04 Dec 2005 12:59:58 GMT, Bonehenge
<Keep_it_in_the_newsgroup_please@aol.com> wrote:
| Quote: | On Sat, 3 Dec 2005 23:45:38 -0800, "Jeff Strickland" <crwlr@yahoo.com
wrote:
Because I sold GPS systems, and have a pretty good idea of how the system
actually works. It's all based on known mathematical formulas and a minimum
of 4 (four) satellites. It's easy to guage the speed.
The main issue with speed was sampling rate, or the number of times in
a time period that the unit calculated a position.
My old Garmin GPS II and III (early and mid 90's vintage) would lag
the speedo quite a bit during spirited acceleration and deceleration.
The eTrex Vista and aviation GPSMAP 196 I have are better, while my
GPSMAP 60CS has an almost non-existent lag. The lag is evident as the
GPS displays below the speedo while accelerating, and above the
speedo's indicated speed while decelerating.
Slamming on the brakes with the older units might have you at a dead
stop with the GPS reading 4-5 MPH for a second or two, or maybe a 5
MPH reading, then a 2, then a 0. Accelerating down a runway in a jet
might see it lag behind 10-20 MPH.
These examples were all with selective availability turned off.
At a relatively constant speed, I've never seen a GPS that wasn't dead
on compared to a timed distance.
|
Appears to work pretty well on cruise missiles.... <g>
--
Scott in Florida |
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Bonehenge
Guest
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| Posted:
Mon Dec 05, 2005 1:30 am Post subject:
Re: speedometer not properly calibrated |
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On Sun, 04 Dec 2005 09:09:48 -0500, Scott in Florida
<JustAsk@Florida.com> wrote:
| Quote: |
Appears to work pretty well on cruise missiles.... <g
|
I'm not so sure a cruise missile needs to know how exactly fast it's
going. The missile needs to know how to get to where it wants to go,
and when it's arrived. |
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Scott in Florida
Guest
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| Posted:
Mon Dec 05, 2005 2:03 am Post subject:
Re: speedometer not properly calibrated |
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On Sun, 04 Dec 2005 19:30:29 GMT, Bonehenge
<Keep_it_in_the_newsgroup_please@aol.com> wrote:
| Quote: | On Sun, 04 Dec 2005 09:09:48 -0500, Scott in Florida
JustAsk@Florida.com> wrote:
Appears to work pretty well on cruise missiles.... <g
I'm not so sure a cruise missile needs to know how exactly fast it's
going. The missile needs to know how to get to where it wants to go,
and when it's arrived.
|
Last I checked....all those factors enter into the equation.
http://howthingswork.virginia.edu/journal/Article1.3.pdf
--
Scott in Florida |
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Jeff Strickland
Guest
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| Posted:
Mon Dec 05, 2005 4:28 am Post subject:
Re: speedometer not properly calibrated |
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|
"Bonehenge" <Keep_it_in_the_newsgroup_please@aol.com> wrote in message
news:ivo5p1hv9ni6sa8p6qafktemtal4a187s0@4ax.com...
| Quote: | On Sat, 3 Dec 2005 23:45:38 -0800, "Jeff Strickland" <crwlr@yahoo.com
wrote:
Because I sold GPS systems, and have a pretty good idea of how the system
actually works. It's all based on known mathematical formulas and a
minimum
of 4 (four) satellites. It's easy to guage the speed.
The main issue with speed was sampling rate, or the number of times in
a time period that the unit calculated a position.
My old Garmin GPS II and III (early and mid 90's vintage) would lag
the speedo quite a bit during spirited acceleration and deceleration.
|
Well of course it would lag under those conditions, but the point was that
the OP wanted to know if his speedo was telling him the truth. When the
speed is rapidly changing, the truth is a variable and anything the speedo
says is useless. The way to get relaible information from a speedo is to
level off on the speed changes, and a GPS works fine to display constant
speeds. |
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Bonehenge
Guest
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| Posted:
Mon Dec 05, 2005 5:10 am Post subject:
Re: speedometer not properly calibrated |
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On Sun, 4 Dec 2005 14:28:52 -0800, "Jeff Strickland" <crwlr@yahoo.com>
wrote:
| Quote: | The way to get relaible information from a speedo is to
level off on the speed changes, and a GPS works fine to display constant
speeds.
|
I TOTALLY agree. <G>
Sorry if you took it any other way. I just though the information
might be useful to the OP. |
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Gord Beaman
Guest
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| Posted:
Mon Dec 05, 2005 7:09 am Post subject:
Re: speedometer not properly calibrated |
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"Jeff Strickland" <crwlr@yahoo.com> wrote:
| Quote: |
GPS is close enough to check a speedo.
How do you know?
|
snip
| Quote: | Now, the GPS systems
are accurate to within 3 meters (about 10 feet), but this degree of accuracy
has nothing to do with the speed.
|
Why not?
--
-Gord.
(use gordon in email) |
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Gord Beaman
Guest
|
| Posted:
Mon Dec 05, 2005 7:12 am Post subject:
Re: speedometer not properly calibrated |
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Bonehenge <Keep_it_in_the_newsgroup_please@aol.com> wrote:
| Quote: | On Sat, 3 Dec 2005 23:45:38 -0800, "Jeff Strickland" <crwlr@yahoo.com
wrote:
Because I sold GPS systems, and have a pretty good idea of how the system
actually works. It's all based on known mathematical formulas and a minimum
of 4 (four) satellites. It's easy to guage the speed.
The main issue with speed was sampling rate, or the number of times in
a time period that the unit calculated a position.
My old Garmin GPS II and III (early and mid 90's vintage) would lag
the speedo quite a bit during spirited acceleration and deceleration.
The eTrex Vista and aviation GPSMAP 196 I have are better, while my
GPSMAP 60CS has an almost non-existent lag. The lag is evident as the
GPS displays below the speedo while accelerating, and above the
speedo's indicated speed while decelerating.
Slamming on the brakes with the older units might have you at a dead
stop with the GPS reading 4-5 MPH for a second or two, or maybe a 5
MPH reading, then a 2, then a 0. Accelerating down a runway in a jet
might see it lag behind 10-20 MPH.
These examples were all with selective availability turned off.
At a relatively constant speed, I've never seen a GPS that wasn't dead
on compared to a timed distance.
|
Thank you sir...
--
-Gord.
(use gordon in email) |
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Gord Beaman
Guest
|
| Posted:
Mon Dec 05, 2005 7:28 am Post subject:
Re: speedometer not properly calibrated |
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Bonehenge <Keep_it_in_the_newsgroup_please@aol.com> wrote:
| Quote: | On Sun, 4 Dec 2005 14:28:52 -0800, "Jeff Strickland" <crwlr@yahoo.com
wrote:
The way to get relaible information from a speedo is to
level off on the speed changes, and a GPS works fine to display constant
speeds.
I TOTALLY agree. <G
Sorry if you took it any other way. I just though the information
might be useful to the OP.
It was and thanks...also thanks to Scott for the data... |
--
-Gord.
(use gordon in email) |
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Jeff Strickland
Guest
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| Posted:
Mon Dec 05, 2005 9:19 am Post subject:
Re: speedometer not properly calibrated |
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"Gord Beaman" <gord@islandtelecom.com> wrote in message
news:hbg4p19a7ek4nvv1c07qpccr4feg85ntii@4ax.com...
| Quote: | "Jeff Strickland" <crwlr@yahoo.com> wrote:
GPS is close enough to check a speedo.
How do you know?
snip
Now, the GPS systems
are accurate to within 3 meters (about 10 feet), but this degree of
accuracy
has nothing to do with the speed.
Why not?
|
The calculations that tell you where you are were muddled up on purpose so
the device couldn't be used as a triggering device. Somebody realized that
if the device was inaccurate to 30 feet, then the device to be triggered
would simply go off 30 feet away, and in the grand scheme of things, this
wouldn't make much difference. They changed the calculations a few years ago
to give more accurate location data.
Speed data is a bit different. This measures the difference in the location
from calculation to the next, and displays the change in location over time
as a speed. You are going 60 here, and 30 feet away so the speed accuracy
hasn't changed.
GPS works by the receiver finding a minimum of 4 satellites. Withoug getting
too precise on the process, the satellites are all synched to the same
clock, and they give a pulse at the same time. The receiver simply measures
the time it takes for the pulse to get to it, and then calculates where it
is on the ground by how long the various signals take to get to it. The
speed of the signal is a known condition. For example, radar waves take
something like .0037 seconds to go a mile, so if it takes .0074 seconds for
the receiver to see the wave, then the target is 1 mile away -- .0037 to go,
and .0037 to come back.
GPS is not radar, duh!, but the science is similar -- the pulse takes a
known amount of time to travel a known distance, so if we measure the
precise time it takes for the signal to get here then we know how far the
transmitter is, and with 4 transmitters in space -- there are 24 of them, I
think, but we only need 4 to have good data -- then we can measure how far
we are from each of them and since we know where they are, we end up with
where we are. Speed is merely a calculation based on how long it takes to
get from one location to the next. |
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