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Gpsrs Vs Speedometers


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We got into caching about a year after buying a Garmin iQue, which we love because it gets us safely to all sorts of out-of-the-way spots. I've used it to navigate around the UK a lot but only recently really watched the display when someone else was driving. The speedo on our car was showing 30mph exactly - the iQue reckoned 28 mph. Later - at 70mph on the speedo it was 65 mph on the iQue.

 

As a techoholic I choose to believe the GPS. My theory is that the car manufacturers deliberately calibrated the speedo a bit low so they're not sued if someone gets a speeding ticket when the dial says they're under the limit.

 

What do you folks think?

 

Deb @ Cunning Planners

 

PS and yes, we now own 3 GPSrs - didn't want to risk the iQue in the hands of the junior members of the team while out walking - "oops, sorry Mum, the screen sort of smashed when I threw it over the stile..." - no excuse for the third one B)

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We got into caching about a year after buying a Garmin iQue, which we love because it gets us safely to all sorts of out-of-the-way spots. I've used it to navigate around the UK a lot but only recently really watched the display when someone else was driving. The speedo on our car was showing 30mph exactly - the iQue reckoned 28 mph. Later - at 70mph on the speedo it was 65 mph on the iQue.

 

As a techoholic I choose to believe the GPS. My theory is that the car manufacturers deliberately calibrated the speedo a bit low so they're not sued if someone gets a speeding ticket when the dial says they're under the limit.

 

What do you folks think?

 

Deb @ Cunning Planners

 

PS and yes, we now own 3 GPSrs - didn't want to risk the iQue in the hands of the junior members of the team while out walking - "oops, sorry Mum, the screen sort of smashed when I threw it over the stile..." - no excuse for the third one B)

You are correct in your assumptions...the speedo always reads fast.... B):oB):DB)

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GPS is much more accurate than a car's speedometer.

 

Looking at the Magellan manual, I see that they claim its accuracy is 0.1mph. Having closely examined some of the other accuracy claims by the manufacturer of this machine, I'm sure they are quite right.

 

Car speedometers are biassed towards over-reading the actual speed because of a government requirement that they never under-indicate the actual roadspeed of the car.

Edited by The Forester
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I agree that GPS is much more accurate. My speedo over-reads by 10%. This is my main defence whenever my wife says "not too fast" B)

 

Another way to check is to use the measured miles which are on many A roads. These are indicated by a sign of a small yellow circle at the start, a red quarter, then a half, then 3/4 then red circle after 1 mile. If you time yourself over the distance (at a steady speed), you can work out your speed and compare it to your speedo and GPS.

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Another way to check is to use the measured miles which are on many A roads.

The blue/white kilometre posts on motorways are quite accurate too. They are set every 100 metres.

 

Mind you, holding your speed exactly on your nominal speed much better than ±0.5mph for a kilometre or more requires the throttle control skills of a Red Arrows pilot!

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Trust the GPS. In my kit car days a speedo calibrator confirmed that they must read exactly or below the actual speed of the car. When you take into account tyre pressures etc that can also affect your speed readout they have to err on this side.

If you ever see inside a police vehicle most have a speed corrected speedo on the dash to confirm thier actual speed.

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Easy way to check. Go through a speed camera set for 50. At say 70 on your GPS and see what the speed was on the photo. Those cameras are suposed to be very accutate.

And you get a bonus Virtual Cache in the process!

 

Although supposedly only 1 in 8 cameras is loaded, so you'd better do it nine times just to make sure!

Got that cache.

 

There is a tale, I don't know if it's true, about a fella who went through a speed trap very carefully below the 30 limit and got flashed.

 

He thought he would go back and return through it at an even lower speed to show that the camera wasn't working and got flashed at 20mph and so he returned at 10mph and got flashed again.

 

He was satisfied that if the tickets arrived they would show that the camera was faulty.

 

A week later he recieved three summonses for not wearing a seatbelt!

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When you take into account tyre pressures etc that can also affect your speed readout they have to err on this side.
Variation in tyre circumference due to wear also has to be accommodated.
If you ever see inside a police vehicle most have a speed corrected speedo on the dash to confirm thier actual speed.
Is it correct these have to be recalibrated every week due to the tyre variations?
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When you take into account tyre pressures etc that can also affect your speed readout they have to err on this side.
Variation in tyre circumference due to wear also has to be accommodated.
If you ever see inside a police vehicle most have a speed corrected speedo on the dash to confirm thier actual speed.
Is it correct these have to be recalibrated every week due to the tyre variations?

That's one of the reasons for the measured mile markers

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I use my GPSr as a speedo on a kite buggy, and looked into this question

 

car speedos are inacurate because they measure speed at the gearbox, then convert it into road speed with a bit of math, consequently they are horribly inaccurate and as already said, they tend to be set to measure over 'on the safe side'

 

GPSr measure speed by taking the first integral of the distanced moved, which it calculates by looking at the position at time 1 and the position at time 2 etc this means that the accuracy is dependant wholly on the accuracy of the position readings, and yes, it sucks as a speedo when driving through a dense forest

 

This also means that GPSr suffers from lag, it never tells you your current speed, but in fact the average speed over the last few cycles (you can guess the cycle time by watching how quickly it updates under heavy acceleration and deceleration conditions)

 

The ratio of EPE (Estimated Positional Error) to actual speed is important too, a low EPE at high speed will give great results, where a high EPE, at low speed will suck.

 

Consequentially, with a GPSr you find that the accuracy as a speedo increases wth the speed and distance you are going, intitially at slow speeds and short distances it sucks, likewise when slowing down and stopping it also sucks, however, in a clear area, with 8-12 good satelites and a low EPE, travelling at a steady high, speed, then it is very accurate

Edited by -Phoenix-
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You are correct in your assumptions...the speedo always reads fast.... 

 

--------------------

 

Cheers,

Dirk.

 

Not true me finks.......my old cavalier says about 80 when the GPSr says 73, BUT our Citroen Xantia says 70 when the GPSr says 73 :rolleyes:

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You are correct in your assumptions...the speedo always reads fast.... 

 

Not true me finks.......my old cavalier says about 80 when the GPSr says 73, BUT our Citroen Xantia says 70 when the GPSr says 73 :lol:

 

You've got a problem there as an under-reading speedo is illegal under the Construction & Use Regulations. If you're not using non-standard wheels/tyres I'd say you've got a faulty speedo :lol:

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Not quite correct, in a geared transmission, the only point where slip is possible is at the clutch, and since the speedo is after the clutch, then the only other possible point where slippage may occur yet not be reflected in the speedo is at the tyres, where you will indeed get increased slippage, especially under acceleration and decelleration.

 

Also, there is a known degree of non linearity in the real-world situation bewtween gearbox and roadspeed, but invariably the speedo math is a simple linear equation offset to be 'accurate', 'ish' over the normal driving range (20-100), consequently when you exceed this range then you are indeed entering the realms of inaccuracy.

 

Remember, proportionally, 240 reading 210 is the same as 80 reading 70, which is only margianlly different from that reported on a cavalier.

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However if your speedometer is reading higher I would trust it. Saves the speeding ticket, unless you really need the ten points for cacher of the month! :o

Not only that, I can't watch the road, drive, stop the kids squabbling and watch 2 different speed indicators -at the same time!

 

(Thinks - wonder whether anyone has ever successfully defended a speeding offence by quoting "trying not to kill the kids" as mitigation)

 

So as not to go completely off-topic, thanks to all for the fascinating information and insights. I figured there must be some leeway on both systems, but hadn't considered tyre pressure.

 

Cheers

Deb

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As the others have said, your car speedo has to read higher than actual speed due to regulations to allow for discrepencies, ie: new tyres compared to old tyres. The speedo will always show the same speed due to being taken from the g/box, but the actual vehicle speed will change depending on the circumference of the tyres (depending on whether they're new or old).

 

I've just got a Garmin eTrex, and also have a Pocket PC GPS, and both show 65 where the car speedo shows 70, and 28 when the car shows 30.

 

I'm a bus engineer and did an experiment with my new GPS the other day to check it's accuracy. The buses have a speedo / tachograph which is calibrated on a rolling road, so should be very accurate when working correctly. I held the bus at 30mph, and the GPS said exactly the same, not slightly less as it does against a car speedo.

 

So I think you can take it that a GPS is very accurate as a speedo (at a steady speed).

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I'm a bus engineer and did an experiment with my new GPS the other day to check it's accuracy. The buses have a speedo / tachograph which is calibrated on a rolling road, so should be very accurate when working correctly. I held the bus at 30mph, and the GPS said exactly the same, not slightly less as it does against a car speedo.

 

So I think you can take it that a GPS is very accurate as a speedo (at a steady speed).

Surely, if you were on a rolling road, the GPS would give a speed reading of zero? :huh:

 

(Sorry, I couldn't help myself!)

Edited by NickPick
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Surely, if you were on a rolling road, the GPS would give a speed reading of zero?    :huh:

 

(Sorry, I couldn't help myself!)

Are you sure your name's NickPick and not Nitpick...lol :unsure:

 

The speedos are calibrated on a rolling road by an authorised company. I was driving on the road / highway, and kept the bus at a steady speed at various speeds, and checked the GPS said the same.

 

Clear enough now...lol :huh:

Edited by HD462
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