GPS, Car speedo, or roadside speed indicator. Which do I believe?

GPS, Car speedo, or roadside speed indicator. Which do I believe? I recently bought a Talex speed camera location device.It works by locking onto 4 to 6 GPS satellites so it knows where I am.

It also gives me an indication of the speed at which I am travelling. This speed is consistently 3mph BELOW the speed indicated on the car speedometer.In addition, near my house is a road sign that measures the speed of approaching vehicles, presumably using a laser, utilising the doppler effect. This sign usually gives a reading about 1mph above the speed on the GPS system.

Which of these three devices should I believe...? Asked by cmansell 41 months ago Similar questions: GPS Car speedo roadside speed indicator Consumer Electronics > Car Electronics & GPS.

Similar questions: GPS Car speedo roadside speed indicator.

Consumer Electronics > Car Electronics & GPS.

GPS Your GPS tracks your location with an accuracy of 3 meters. So long as its internal clock is accurate, it will give you the most accurate reading, even at relatively low speeds. I have a handheld GPS, and it even measures my speed when I'm walking.

The roadside radar device (it's not laser) does not read in fractions, so it will round up to the nearest 1 MPH. Also, the radar horns in those things aren't state-of-the-art. The cops have better ones, and they are required to calibrate them daily.

Even then, they can only approximate the speed of an approaching vehicle because of cosine error. The closer you are to the horn, the more the error. However, it's going to give you a more accurate reading than your car speedometer.

Car speedometers use a sensor on the driveshaft to measure your speed as a function of wheel revolution. Because the tires are made out of rubber, their diameter varies with speed, load, and temperature, even when inflated to their recommended pressure. You'll never get an accurate reading from a car speedometer; if you have ever seen a test vehicle on a track, you might have seen something like this trailing it: It's called a fifth wheel speedometer, and it was designed to compensate for the inaccuracies I mentioned.

Like you, I was surprised at how much difference I saw between my car's speedometer and my GPS unit, although it's better on the car I have now than it was on my last car. If your car speedometer is reading higher than your GPS, it may be that your tires are slightly undersize for your car, which is the problem I had on mine. As long as it's not telling you you're going 35 when you're really going 50, you shouldn't have a problem.

Sources: Experience .

The gps for accuracy, the speedo for legal purposes Car speedos are routinely set to show a little too much, in order for the car manufacturer to avoid being swamped by irate drivers blaming them for their speeding tickets. The road sign is designe with the explicit purpose of getting people to slow down, so that one is probably also low. The gps(so far) haven't been corrupted in this way so it will likely be closest to the truth.

However, if you get pulled over for speeding you won't get anywhere by claiming "OK by the gps". Accurate as it may be, the gps hasn't been calibrated - but the officer's measuring device has.

Complimentary speed' may be part of the answer. I heard of something -in the pub- so it is not especially scientific but completely anecdotal. (Also this is in London, England, not the USA) However- someone was going on about 'complimentary speed' a concept I have head neither before nor since.

He claimed that car manufacturers would set the speedometer to register a speed of up to ten percent higher than your true speed, as a safety feature. In the papers, there are reported problems over here with satellite navigation systems. These can be amusing if you have an unpleasant sense of humour, as they often come with 'schadenfreude' photographs, of articulated trucks wedged between farm track walls when they thought they were going to be going down a main road; with accompanying tales of tour buses going to Edinburgh but arriving in Aberdeen, etc.There have been massive traffic jams near where I live, where SATNAV has funnelled hundreds of vehicles into a relatively minor back street as some kind of short cut.

In brief, all the bugs in this system may not have been ironed out yet. Apropos of nothing at all, I saw a Jamaican nighclub comedian going on about how he wanted an ethnic specific SATNAV system. He thought a relaxed Rastafarian voice telling him 'Left or right, just as so long as you stay cool and have a nice day.

' Sources: baseless pub and newspaper gossip without a shred of evidence .

1 OMG, STOP WATCHING THE GIZMOS AND DRIVE! Really, the only one you need to worry about is the cop with the radar gun...and then only if you are speeding! Just kidding.

Usually, speedos etc. Operate on the "close enough" theory...close enough to observe the speed limit. The indicator sign may even be set to display differently just to encourage you to drive slower.Is there really a reason to need to know the EXACT speed you are going? If there is, I vote for the GPS and the satellites.

OMG, STOP WATCHING THE GIZMOS AND DRIVE! Really, the only one you need to worry about is the cop with the radar gun...and then only if you are speeding! Just kidding.

Usually, speedos etc. Operate on the "close enough" theory...close enough to observe the speed limit. The indicator sign may even be set to display differently just to encourage you to drive slower.Is there really a reason to need to know the EXACT speed you are going? If there is, I vote for the GPS and the satellites.

2 I vote for the GPS too. The speedometer doesn't want to be blamed for a speeding ticket and the radar sign is there to make you slow down.

I vote for the GPS too. The speedometer doesn't want to be blamed for a speeding ticket and the radar sign is there to make you slow down.

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I am looking for a gps system for my car. Not too expensive. Recommendations?

Sites for good prices? Thanks!

My dad would like a GPS for his birthday, would like it to be for car and able to take into the woods what should I look.

What kind of gps car charger do I need for the lowrance I way 250c.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.