Which Device is more accurate on speed?

[Ones]

Which is more accurate on speed?
Speedometer it self, GPSr with SiRF3, GPSr with MTK chip set, Laser gun or Radar gun? :?

• I asked the Question sometime relaying on GPSr for the speed reading will get some of us in to trouble.
• As for Laser Gun
Which laser gun is more accurate and fool proves? :?

[jerry540i]

Your speedometer is probably inaccurate. Alot of manufacturers purposely set their speedometers on the high side to slow you down. I know this is the case with BMW's at the higher speeds.

I don't know much about GPS, although some people swear by the Sirf3 chip set, this is probably just preference (kinda like radar detector preferences I guess.) Anyway, GPS is accurate, provided you don't lose the signal.

Both radar and laser are proven technologies and are extremely accurate if used correctly.

Radar is prone to cosine error, whereby the speed displayed will be lower than the actual speed of the car, the greater the angle the radar gun is shot at. Radar can also show an incorrect speed because of panning error and shadowing error.

Laser is very accurate. However you can also get a laser gun to incorrectly display a speed ie: pannning error from hood to windshield.

Which gun is the most accurate? All of them work very well in acquiring your speed. Some are easier to jam (Kustom Pro Lasers, LTI's products) while other lasers have software that recognizes when it is being jammed (Laser Atlanta for example)...but all are very accurate, and fool proof if used correctly.

J/

[TSi+WRX]

Your speedometer is probably inaccurate. Alot of manufacturers purposely set their speedometers on the high side to slow you down. I know this is the case with BMW's at the higher speeds.

Not only that, but any number of other error-inducing factors - such as above-or-below factory-"sized" tire dimensions - can also cause significant errors when it comes to the speedometer reading. This even applies to ancillary/auxiliary gauges that either taps the vehicle's speedo signal or uses the vehicle's OBD-II output to display vehicle speed.

Unless the speedo is "certified," it shouldn't be used as a benchmark for true vehicle road-speed.

I'm also unfortunately not very well-versed with respect to GPS. As-such, I have a question on this, myself. ops: :?: :arrow: Does the GPS system have a "sampling rate," and would/could this cause an error?

[ELVATO]

I know that the GPS on my phone has a slow "refresh rate," meaning it updates the speed every half second, to every second. But then again, I don't really use it for the purpose and consider it more like a novelty. Either way, according to this, my speedo is about 2 mph off on the high end.

[Windstorm]

GPS becomes more and more accurate as speed increases. This is because there is always significant error with gps position, usually around a few meters. That's why when you stand still some GPS will still show a few mph of "speed", and those that don't use software smooting to average out the error, but the error is still there.

As your speed goes up the error becomes relatively smaller, becoming insignificant around 20-50 mph (depending on the sample rate and software). Past this point GPS will be more accurate than a speedometer.

Having said that there will always be a lag with GPS - remember it's not a speed measuring system, it calculates speed from position, so it needs a few seconds' worth of position history to calculate a speed. Best is to use a GPS to calibrate your speedometer by driving at a steady speed.

The chipset itself wouldn't make much difference, but the software would. If you use a dedicated GPS unit you don't have much choice for software. However, I've used different software on my pocket pc: some are more aggressive at averaging out GPS errors to give you a smoother readout, others show you closed to the actual calculated speed giving erratic readings at low speed.

[amoney]

[quote="TSi+WRX"]...Not only that, but any number of other error-inducing factors - such as above-or-below factory-"sized" tire dimensions - can also cause significant errors when it comes to the speedometer reading...quote]

Ditto

Not sure by how much but, my SirfIII chipset GPS is a few mph slower than my speedo, and I have + sized my tires, so I think thats pretty telling.

[Roundstone99]

I've always been told radar is the most accurate since it's a direct reflection of the target. Of course the "accuracy" is relative to the radar. A speedometer is going to be the most accurate to the driver.