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post #11 of 22 (permalink) Old 04-26-2004, 05:13 PM
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Reading Topic: My Speedometer is off

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Then you go past a Farva a mile further and he 'says' you were actually doing 81? *What are you going to say? I was only doing 76?...yeah, then he writes you up for 76 instead and saves you $10...big deal
In Texas, a reduction from 81 to 76 would save you $50. You're also MUCH likelier to have a judge just toss it out, since it's only 11 over and it's below the psychologically important 80mph number.

I'd take a $50 reduction and/or thrown out ticket anyday of the week.

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post #12 of 22 (permalink) Old 04-26-2004, 05:25 PM Thread Starter
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Reading Topic: My Speedometer is off

I guess its not THAT big of a deal... I dont speed really, it just bugs me that when the needle is pointing at 65 or 70 that im not actually going 65 or 70

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post #13 of 22 (permalink) Old 04-26-2004, 05:34 PM
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Reading Topic: My Speedometer is off

You really can't be certain.

Like was said, if you were going uphill/downhill at all, even a little bit, the numbers would be off.

Or maybe your GPS is off, and it just so happens that your truck was also off the same amount.

That's why cops generally give you 10 over at a minimum.

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post #14 of 22 (permalink) Old 04-26-2004, 05:38 PM Thread Starter
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Reading Topic: My Speedometer is off

Well with my GPS in my partners MSP ... with his smaller winter tires on the gps was reading 2-4 mph slower then his speedometer... he put the stock summer 17's on last weekend and the GPS was accurate to his speedometer for the most part... especially out by my house which is pretty flat... at a constant speed.

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post #15 of 22 (permalink) Old 04-26-2004, 11:37 PM
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Reading Topic: My Speedometer is off

I have been using handheld GPS for a few years now. I had a Honda Civic that would always indicate 2mph faster than the GPS. I even cross-checked a different handheld with similar result. Here is what I know from studying this topic.

The GPS equimpent you own is the limiting factor in the discrepancy, although I do believe that some speedometers are off a bit too. The variables that affects any GPS the most are the speed of light and you moving. Before I explain, I must remind you that there must be at LEAST three satellites within line-of-sight in order to triangulate your geoposition, direction, speed, altitude, time-to-go or whatever. Otherwise your GPS would not even work at all.

As "pchaves" mentioned, the military is indeed capable of more accurate satellite service than the public. This is due to more expensive and advanced GPS equipment which corrects for the errors I am will explain below. The last I heard, the military has GPS accuracy down to a sqare foot or so, while the public is limited to however many square METERS. Oh, did I mention that you get what you pay for too ?

GPS is made possible by light communication with the satellites. Those satellites orbit hundreds of miles high depending on where they are in relation to the horizon (on horizon would be much further than directly above). The amount of time it takes for your GPS to transmit its blip, then for the light to travel to all satellites involved (often as many as 9), those satellites to process and return the pulse of light, and your GPS to process the [up to] 9 incoming signals takes a few moments. Keep in mind, both you and the satellites are in motion so there will be inherent errors from this fact alone. The send/receive process between GPS and satellites is continous and very frequent.

Now, in that extra second of your GPS waiting for replies from all the satellites, you have moved a few extra feet. The satellites receive the signal thinking you are exactly where the signal was sent when in fact that point is slightly behind where you truly are. While the light returns from the satellite you move a little more, and the time delay compounds as the send/receive process continues. The result is the satellites and your GPS think that you are constantly a short distance behind your true position - hence showing a slower than actual speed.

You will notice this effect if you slam on your brakes...pay attention to the speed on the GPS. Even when you reach a complete stop, your speed will show 5 mph or whatever for a second longer. That's a perfect example of how long the send/receive process really takes.

Does this make any more sense now, or did I confuse the heck out of you guys? I hope it isn't too lengthy. See ya!

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post #16 of 22 (permalink) Old 04-27-2004, 12:12 AM Thread Starter
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Reading Topic: My Speedometer is off

Yah, I understand you and totally agree.. however coming from my last vehicle where my GPS and speedometers matched pretty much exactly when holding a constant speed... Its just annoying to me that either now my speedometer isnt as accurate and or the car is somehow affecting the gps..

the software I use wont even begin to fix your position until the GPS is trackingat least 4 sattelites.

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post #17 of 22 (permalink) Old 04-27-2004, 12:40 AM
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Reading Topic: Reading Topic: My Speedometer is off

there is an old thread on this issue and i was the last one to post on that one....ur speedo is wrong and so is everyone elses....i check the odometer and it functions correctly...there is a highway that has a radar that shows how fast your going close to my house the 6 is almost always off by 5mph...since i wasnt sure if that radar was correct i took my mom's car...the radar showed the same speed as her speedo....both cars had the same exact odometer readings..at first i thought the speedo was calibrated for the base 16'' also but thats not it because it would be less than 5mpp difference....someone posted that the european versions with Kmh speedos are under by 3 kmh....so the only thing that made sense to me was this conclusion...the speedos on the 6 are misprinted....in other words everything functions correctly as far as mechanically(needle, odometer) but where the numbers are printed on the gauges should be rotated more to the right...in other words where it shows 60 on ur speedo should be slightly more to the center to the right.....so where it shows 60 should of actually been printed 55....just rotate the speedo numbers to the right by quarter of an inch
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post #18 of 22 (permalink) Old 04-27-2004, 02:55 AM
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Reading Topic: My Speedometer is off

Cops in Michigan generally stick with 5 over. I've never had a ticket for more than 5 mph over the limit, and I've probably been pulled over 15-16 times.
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post #19 of 22 (permalink) Old 04-27-2004, 10:23 AM
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Reading Topic: Reading Topic: My Speedometer is off

So rotate the speedo clockwise 5mph to figure an accurate speed? Does anyone else concur with this? Since I havent had my 6 long enought to find out for sure, I dont know. A very good friend of mine is a police officer - I will have him clock me while we communicate by phone and find this out for sure. I will let you all know in a few days.

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post #20 of 22 (permalink) Old 04-27-2004, 12:10 PM
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Reading Topic: Reading Topic: My Speedometer is off

There is an easy way to calculate all of this..

You can do this yourself or with a friend... But get on the Interstate and travel at a set speed. Have your friend with a stopwatch a start timing when your car passes a mile marker. Let's say you were traveling at 60 MPH. Therefore, if your speedometer is right, it should take you 60 seconds to travel a mile. If that time is off by a few seconds, you know that the speedometer is off.

If it's off, then here are the calculations to do:

5280 ft. / (TIME YOU MEASURED) seconds = YOUR_VALUE
YOUR_VALUE (feet per second) X 60 (seconds in a minute) = YOUR_VALUE (feet per minute)
YOUR_VALUE X 60 minutes in an hour = YOUR_VALUE (feet per hour)
YOUR_VALUE / 5280 feet in mile = TRUE MPH

So.. for example... If I measured that it took me 45 seconds to travel a mile...

5280 / 45 = 117.333... ft per sec
117.333 X 60 = 7040 ft per minute
7040 X 60 = 422400 ft per hour
422400 / 5280 = 80 MPH.

There are a lot of factors for the speedometer to be off... From tire size to even slight tire wear. Gear ratios also affect them too :)

Hope that helps...

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