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Is any of this true?


LostMontanan
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Hey gang! We are in a debate over at a motorcycle forum I belong to. The original topic was about what percentage our speedometers are off compared to a GPS reading. There are a number of answers that make some kind of sense, but this answer here just kind of struck me as possible, but extremely odd. Anybody know if there is any truth to this?

 

Actually due to the fact that the United States owns all the GPS satelites and the fact that we are waging a war, it is extremely possible that your GPS receiver is off by quite a bit. The President has the authority to insert an error into the GPS signal to thwart our enemies who might also be using a handheld unit. Normally GPS systems without an valid key-code have an average error rate of aprox 300 ft Spherical Error Probability (SEP) (basically its accurate to a 300 ft bubble around your location. However, with a valid code, [you'll have to look that up yourself. ] GPS also issues with speed error; similar to our speedometer, the faster you go the more of an error is induced. This insertion of invalid data into a GPS signal is called "Spoofing" (don't know who came up with that name).

 

This is not to say that your GPS is a POS or anything, just don't bet the house on your GPS reading.

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Bogus. Selective Availablity was turn to nil in 2000 (or was it 01) by the orders of President Clinton. SA was used before that time to throw off your position by as much as 300' to protect our interests against missle attack. It was controlled by the US military. The global positioning system is now so intrigal to all aspects of life that turning SA back up again would be distasterous. Planes would likely crash. The only way you would ever see it again is if a nuclear attack had actually been launched.

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SA was turned off at midnight on May 1, 2000. I think there were quite a few parties of celebration in the GPS world.

 

Wikipedia has an interesting article. Spoofing is another matter altogether, I guess the question is why would anyone want to spoof you while you were riding down the road on your motorcycle? I mean unless you were riding beside Fort Knox, or maybe the White House or something... :D

 

It's simple enough to test... find a long flat stretch of interstate with mile markers, set the cruise control on 60 (by your GPS) and see if the markers don't fly by at exactly 60 second intervals. Assuming a few things, including the stability of your cruise control, and the competence of your local highway department surveyers! :o

 

Theron

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I think the GPS units are more reliable than your (or at least my) speedometer.

 

I currently own two Honda automobiles and previously had a third. Every external device that I have ever used to measure my speed in a Honda ( several GPS units as well as many radar speed readout installations on the side of the road ) has indicated my speed was about 4-5 MPH slower than what my speedometer indicates.

 

I'm curious if others have done similar "calibrations" and if this is just a Honda thing or if other manufacturers have similar issues.

 

TM

Edited by tomm2
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Many, if not most, car makers deliberately put error in the speedometer, and most speedometers read a few mph higher than the actual speed. This prevents lawsuits caused by speeding tickets received while the speedo was reading near the legal speed. Also, the input is wheel rotations, so having your tires underinflated, and worn tires, will also cause erroneous readings. The GPS speed is far more accurate than any stock automobile speedometer.

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I think the GPS units are more reliable than your (or at least my) speedometer.

 

I currently own two Honda automobiles and previously had a third. Every external device that I have ever used to measure my speed in a Honda ( several GPS units as well as many radar speed readout installations on the side of the road ) has indicated my speed was about 4-5 MPH slower than what my speedometer indicates.

 

I'm curious if others have done similar "calibrations" and if this is just a Honda thing or if other manufacturers have similar issues.

 

TM

I have read for years that american manufactures calibrate the speedometers about two mph fast. That holds true with our Pontiac Vibe, but my Chevy Impala is right on the mark with my 2610.

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I think the GPS units are more reliable than your (or at least my) speedometer.

 

I currently own two Honda automobiles and previously had a third. Every external device that I have ever used to measure my speed in a Honda ( several GPS units as well as many radar speed readout installations on the side of the road ) has indicated my speed was about 4-5 MPH slower than what my speedometer indicates.

 

I'm curious if others have done similar "calibrations" and if this is just a Honda thing or if other manufacturers have similar issues.

 

TM

 

My 2006 Civic is pretty much on the mark. It's never shown more than a few tenths of a mile difference, and since the car has a digital readout that shows only full miles per hour, I'm guessing it's pretty accurate.

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This insertion of invalid data into a GPS signal is called "Spoofing" (don't know who came up with that name).

 

Spoofing is when a third party generates false (fake) GPS signals, which mimic actual signals of real satellites. This would make the GPSr generate a false position. This is relatively easy since the signal definitions are known.

 

Anti-spoofing uses an encrypted channel. This channel is more accurate, and encrypted by keys available from the military. The encryption theoretically makes it impossible (or at least very hard) to spoof a real signal.

 

(Edited to clean up quoting)

Edited by BBWolf+3Pigs
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I would put more faith in your GPS than the speedometer.

 

The speedometer is affected by too many factors, starting with the computations by the vehicle engineers as to gearing ratios, manufacturing quality of speedometer and related parts, even down to tire profile (slightly).

 

Did you know that changing the tire size or wheels on your car (say from stock to low-pro) will throw the speedometer off? If you make a drastic change, such as when lowering, or adding lift kits, you need to have the ratios recomputed and change a gear in the input to the speedo to correct it, if you desire it to read correctly.

 

The exact percentage your speedo is off depends on modifications made. From the factory, probably fairly close, not more than 5% off. (IE 57 instead of 60) and not the same % in each gear.

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Spoofing is when a third party generates false (fake) GPS signals, which mimic actual signals of real satellites. This would make the GPSr generate a false position. This is relatively easy since the signal definitions are known.

 

Anti-spoofing uses an encrypted channel. This channel is more accurate, and encrypted by keys available from the military. The encryption theoretically makes it impossible (or at least very hard) to spoof a real signal.

 

So the OP in the quote I provided was sort of correct....thanks for the info everyone.

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I would put more faith in your GPS than the speedometer.

 

The speedometer is affected by too many factors, starting with the computations by the vehicle engineers as to gearing ratios, manufacturing quality of speedometer and related parts, even down to tire profile (slightly).

 

Did you know that changing the tire size or wheels on your car (say from stock to low-pro) will throw the speedometer off? If you make a drastic change, such as when lowering, or adding lift kits, you need to have the ratios recomputed and change a gear in the input to the speedo to correct it, if you desire it to read correctly.

 

The exact percentage your speedo is off depends on modifications made. From the factory, probably fairly close, not more than 5% off. (IE 57 instead of 60) and not the same % in each gear.

I learned when I did a sports car rally with a stock speedometer that tire size is affected by tire pressure. You can change the speedometer by changing tire pressure. Pressure changes with temperature.

 

As far as the GPS speed, the GPS measures horizontal speed and the speedometer measures the amount of road traveled. The road is longer than the horizontal distance when you go over a hill.

Edited by John E Cache
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Hey gang! We are in a debate over at a motorcycle forum I belong to. The original topic was about what percentage our speedometers are off compared to a GPS reading. There are a number of answers that make some kind of sense, but this answer here just kind of struck me as possible, but extremely odd. Anybody know if there is any truth to this?

 

Actually due to the fact that the United States owns all the GPS satelites and the fact that we are waging a war, it is extremely possible that your GPS receiver is off by quite a bit. The President has the authority to insert an error into the GPS signal to thwart our enemies who might also be using a handheld unit. Normally GPS systems without an valid key-code have an average error rate of aprox 300 ft Spherical Error Probability (SEP) (basically its accurate to a 300 ft bubble around your location. However, with a valid code, [you'll have to look that up yourself. ] GPS also issues with speed error; similar to our speedometer, the faster you go the more of an error is induced. This insertion of invalid data into a GPS signal is called "Spoofing" (don't know who came up with that name).

 

This is not to say that your GPS is a POS or anything, just don't bet the house on your GPS reading.

What I want to know is what you are going to post in response to that idiot poster? :P

 

Just be careful, you never know how someone might react . . . :(

 

:lol:

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The exact percentage your speedo is off depends on modifications made. From the factory, probably fairly close, not more than 5% off. (IE 57 instead of 60) and not the same % in each gear.

 

Bikes really are generally 10% off (high), and since they drive the odo/speedo from the front wheel the percent will the same regardless of gear.

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It is a significante differense between how accurate the car manufactures set up their speedometer. I have a couple of cars and none of them are totaly correct. In my Land Rover, it says 100 km/t (62 miles), when its only 90 km/t (55 miles) on my 60cs. When my Audi Allroad says 100 km/t, my 60cs says 98 km/t. To my knowledges, VAG cars is known to be rather accurate.

 

Funny story; A couple of friends of my sailed around southe America under the Golf war. Their gps suddenly placed them somewhere in Brasil.

Edited by villmarkenskongle
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I think it's unlikely that even if SA were turned back on, that it would affect your GPS speed reading. A modern GPS doesn't use a position calculation to determine the speed value. It's done by detecting the Doppler shift in the signals. Determining the speed by the Doppler method is supposed to be accurate to 1/10th of 1MPH.

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The exact percentage your speedo is off depends on modifications made. From the factory, probably fairly close, not more than 5% off. (IE 57 instead of 60) and not the same % in each gear.

 

Bikes really are generally 10% off (high), and since they drive the odo/speedo from the front wheel the percent will the same regardless of gear.

 

 

THis practice is loooooooong gone. Most bikes take their speed off of a fixed rotational device inside the motor such as a certain transmission parts that turn at same speed no matter what changes you do to exterior of bikes, such as a different size sprocket, different tires or such. It is all digital at this point. My Yamaha at 100mph is off by exactly 10%. It makes sense to be inaccurate toward high side since legal liability issues. Also, sport bike at least, at sold on the count of absolute speed, so "optimistic" speed will only help sales. Nevertheless, bike speedos are much more inaccurate compared to cars and GPS accuracy is light years ahead.........

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I am in aviation field and I can not imagine this SA being turned on when sooo many airplanes depending on it.......Makes no sense at all. IFR flight without GPS guidance is unimaginable at this point. I work for Hawker/Beechcraft by the way and all our aircraft depend on GPS so if you guys know otherwise, let me know and I can relay the info to Whichita, KS.

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As far as the GPS speed, the GPS measures horizontal speed and the speedometer measures the amount of road traveled. The road is longer than the horizontal distance when you go over a hill.

 

I'm fairly sure this is incorrect. I fly Garmins in high-altitude balloons for tracking, and the GPSr always returns three-dimensional speed. After the balloon bursts at altitude (90,000+ feet), the payload drops like a rock and approaches 90-100 mph, while not moving horizontally at all.

 

Also, the recorded track is always much longer that the ground track. For example, our last flight reached 94,000', and the total distance from the launch to landing was 20 miles. The track length as recorded by the GPSr was 53 miles, accounting for the distance up and down.

 

So at least my Legend and Yellow record a 3-D track.

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I think the GPS units are more reliable than your (or at least my) speedometer.

 

My GPSr shows my max speed as 244 mph. I am 100% certain that I never traveled at that speed with the GPSr on.

The GPS max speed can get pretty interesting. Users have reported upwards of 900 mph. IIRC, NeoGeo has topped 1,000. These "max speeds" occur rarely, but occasionally, due to a bug that allows the receiver that has lost its fix, drifted, and suddenly regained its fix, to interpret the change in position as a fast movement. What Prime Suspect says, though, is true. The current speed indicator that updates once per second on most GPSrs is measuring dopplar shift in signals it receives, and is accurate to about .1 mph. A properly functioning GPSr, in addition to being the most accurate time piece you own, is also likely the most accurate speedometer you'll own, unless you happen to have scientific speed measuring equipment in your garage.
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THis practice is loooooooong gone. Most bikes take their speed off of a fixed rotational device inside the motor such as a certain transmission parts that turn at same speed no matter what changes you do to exterior of bikes, such as a different size sprocket, different tires or such. It is all digital at this point.

 

Those of us that ride dualsports from the Big Four are generally riding bikes effectively unchanged from the 80s (or in one case, from the mid90s).

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I think the GPS units are more reliable than your (or at least my) speedometer.

 

I currently own two Honda automobiles and previously had a third. Every external device that I have ever used to measure my speed in a Honda ( several GPS units as well as many radar speed readout installations on the side of the road ) has indicated my speed was about 4-5 MPH slower than what my speedometer indicates.

 

I'm curious if others have done similar "calibrations" and if this is just a Honda thing or if other manufacturers have similar issues.

 

TM

 

Same with my Hondas. However, I have been in American cars that show as much as 15 kph too slow. Keep in mind that worn tires will cause your speedo to over-read. Also note that the odometer and speedo are independant of each other; how, I do not know. My ex's 2000 Civic showed an intriguing error on her speedo: accurate below 80 kph and then off by as much as 15 to fast by 110kph. It wasn't a fixed error nor a percentage of the speed. Interestingly though, her odometer was dead on always.

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I read they can still turn on selective availbility on a regional basis if a national emergency would arise.

SA doesn't really lend itself to being turned on regionally; the satellite footprints are too big.

 

The idea is that they would jam the civil signals and still be able to use the military signals.

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THis practice is loooooooong gone. Most bikes take their speed off of a fixed rotational device inside the motor such as a certain transmission parts that turn at same speed no matter what changes you do to exterior of bikes, such as a different size sprocket, different tires or such. It is all digital at this point.

 

Those of us that ride dualsports from the Big Four are generally riding bikes effectively unchanged from the 80s (or in one case, from the mid90s).

 

Ahhhh, That makes sense......I ride a newer bike and after messing with the sprocket sizes (just a bit smaller for more top speed - track purpose) My speedo was way off. Even race tires due to higher profile for greater lean angle will really mess the speedo up....

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Same with my Hondas. However, I have been in American cars that show as much as 15 kph too slow. Keep in mind that worn tires will cause your speedo to over-read. Also note that the odometer and speedo are independant of each other; how, I do not know. My ex's 2000 Civic showed an intriguing error on her speedo: accurate below 80 kph and then off by as much as 15 to fast by 110kph. It wasn't a fixed error nor a percentage of the speed. Interestingly though, her odometer was dead on always.

It's all about the lawyers....

 

The manufacturer will get sued if they err on the side of making you speed when it's telling you are not, because you will get speeding tickets. And the manufacturer will also get sued if they make a faulty odometer since the value of a used car is heavily dependent on the number of miles it has been driven. So they err on the side of caution for the speed, but they can't be wrong at all on the distance. If they are, then either the buyer or the seller would have an unfair advantage.

 

IANAL, of course, this is my opinion only, but it sure makes sense to me! :)

 

Theron

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