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so lately i have been doing some reasearch on speedometers and gps's. well pretty much everywhere i look i see that a gps is more accurate than speedometers.

but how accurate are they both really?

just like speedometers you would think that they get more inaccurate at higher speeds or am i wrong in thinking that?

is there a way you can get your speedometer more accurate at higher speeds?

are some gps's more accurate than others?

is there something more accurate that you can use inside your car that will tell you how fast you are actually going?
 

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Usually the accuracy of the positioning on a GPS is published. Some are very accurate other s are less accurate. For Calculating accurate speed at high speed, the sampling of the GPS has to be quick and frequent along with the processing speed has to be fast.
 

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Speedometers have changed over the years. Before the 60's they were within 10 or 15 mph. From the 60's up until the late 90's most had an error rate of about 10% so at 55 mph you had a range of 50 to 60 mph, most GM gages error ed on the plus side so at 55 you were doing between 50 and 55. And it was linear, at 100 you could be going as low as 90. So at 150 you could be doing as low as 135.

Starting in the late 90's with the advent of the electronic and computer measurements the speedometers are very accurate. I have found that the measurements of todays car match the GPS devices I have. Even the after market Dakota speedometer that is adjustable to tire size sticks right with the GPS readings. The tire technology all so comes in to play as todays tires do not expand with speed as the older tires did, making it easy-er to get an accurate speed reading.
 

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I think a Pitot Tube is still the most accurate means of determining actual speed. Its still used on Airplanes. Besides how cool would it look to have one sticking off you car.
 

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in 2007, i was cruising along in my slightly modded, and slammed c5, i saw the blue lights behind me. I looked down and thought, "****, a 140 mph ticket", thats exactly what the cop showed me on his laser before writting a ticket and sending me thru court system to clear it.
 

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Yeah, it was a solo cop, on a BMW bike, he saw me coming and got out rite behind me, he was pissed and did mention it took him a mile or so to catch up to me. He never had his lights on til he was just behind me. I was actually slowing down, that was during one of my 165 mph runs in an attempt to hit 185. Theres a good stretch out here in hawaii by the old race track were ive done this many times, only that time i got caught.
 

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GPS is probably the most accurate solution that is readily available. The positioning accuracy of a given GPS does not have much affect on speed measurements since any absolute position errors present at a given sample is likely going be present at roughly the same magnitude and direction in the next sample so any errors are canceled out in the speed calculation.

A pitot tube and airspeed indicator is terribly inaccurate and is subject to variations due to air density (pressure, altitude, temperature, humidity) and would only measure speed relative to to the air the vehicle is moving through. Any wind at any given moment would have to be compensated for and, since there is no way to measure that, cannot be reliably accounted for.

The speed measured by my nav program on my Iphone (by GPS) and the speed indicated on my radar detector (also by
GPS) agree exactly. This also agrees exactly with my cruise control set speed as indicated on the DIC in my 99 FRC. My speedometer reads about 1 MPH high at 70 MPH relative to the other three indicators.
 
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