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davedpc

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Is gps in the car an accurate speed measurement or are we all driving slower than necessary? My gps operated radar detector and my hand held spare unit from my boat both read generally about 5mph slower than the speedo. I have tried it in several different cars and in only one did they match at motorway speeds.

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Not always accurate because the GPS update rate is only every couple of seconds, and you can change speed quite a lot in that time. But yes I got similar results, i'm assuming GPS must be more accurate than car speedos, which all seem to overread by 5-10%, on purpose I believe, so no excuse for speeding !

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I would have thought that GPS was likely to be less accurate than your vehicle's speedo because of the signal distances involved with gps! but I could be wrong!
What about the guy that just gone done for speeding at 31mph? he said his speedo was reading just below 30mph and he had a tachometer reading to prove it!

Barry


<hr width=100% size=1>I just want to retire with my boat to the Med!
 
My GPS 72 reads slower than the speedo - but it was much, much slower till I changed from knts to mph - doh

<hr width=100% size=1>Adrian <A target="_blank" HREF=http://www.arweb.co.uk/argallery/kelisha>Kelisha</A> I never snub anyone accidentally
 
Checking the Alfa speedo against my Magellan shows it consistently reads 3MPH high, eg. at a speedo reading of 70MPH GPS show 67 and thats over reasonbly long distances like 2-3 miles. The Axim/Tom-Tom package in the wife Toyota shows a 4MPh the same way and agrees with the Alfa/Magellan difference.

<hr width=100% size=1>Jim

Draco 2500
 
Yes the GPS is rght and the car speedo over-reads (as it is required to by law). On mine the speedo is 4mph faster at 70mph. The position error in GPS doesn't effect it - in my car GPS there is an average speed function so you can put car on cruise control and measure GPS speed over a mile or so, thus making the GPS position error effect near zero. It still reads 4mph less than the car speedo

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In 3 different vehicles my h/h GPS agrees with the speedo at 30mph. At 70mph the GPS varies between 3 & 8mph slower.

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At least someone has some sense. It's a big worry that so many large boat owners don't understand GPS (and car speed measurement) enough to even ask this question, let alone ponder over it.

OK so there are issues regarding how oftenthe GPS takes a new reading, but if your going down a motorway, your speed is relatively constant.

If GPS speed wasn't extreemly accurate, how the hell does one think that it would be capable of calculating your position within a few metres!!??

And people then struggle to compare GPS accuracy with a mechanical system reliant on variables such as tyre pressures, tread wear, analogue electronics, or mechanical dials, and as you say, a legal requirement to make sure they overread rather than under-read!

<hr width=100% size=1><font color=blue> <A target="_blank" HREF=http://www.mboat.org>http://www.mboat.org</A></font color=blue>
 
i tried this in several cars over the past few months and then found out that because i had bigger wheels than standard this also affected my speedo - but i have never heard of anyone who when buying a new set of alloys for a car having their speedo system reconfigured - but overall i take the GPS to be more accurate - especially if you can cahnge the refresh rate

<hr width=100% size=1>I dont care if its a PETROL it rocks
 
Well done Mr whiteley! here here :o))

Regards Rob

<hr width=100% size=1><A target="_blank" HREF=http://www.rejuvanu.com>RejuVanu</A>
 
Really cant see the issue. All of that is pretty basic.
You did forget the algorithms that the GPS uses to calculate velocity however.

Drive through a tunnel at speed and change direction as you come out. As the GPS loses satellite signals, it will average your speed and direction, and continue to give a bearing and speed even though it has no signal.

<hr width=100% size=1>There is no such thing as "fun for the whole family."
 
Take your points Brendan.

The best use for GPS is surely to 'calibrate' your speedo.

This would best be done on a straight section of motorway with no bridges.

Like most here, my VW speedo overreads by about 3 to 4 mph at 70. My last car, a Frontera, overread by about 7 mph at 70 !.

My TomTom kit and Garmin read exactly the same at all speeds.

<hr width=100% size=1><font color=blue> <A target="_blank" HREF=http://www.mboat.org>http://www.mboat.org</A></font color=blue>
 
The other way to do it is to use the police 1 mile mark posts on the motorways and some dual carriageways(these the one with circles on top. marked in yellow and red quadrants, which mark measured mile in quarters) can use stop watch and constant speed. Constant 60 should give you exactly 60 seconds.

They use these to check calibration

<hr width=100% size=1>There is no such thing as "fun for the whole family."
 
Normally peeps who add bigger alloy wheels compensate with lower profile tyres to return to same rolling radius.Lower profiles look good but will shake the fillings out of your teeth.

<hr width=100% size=1>If it aint broke fix it till it is.
 
when car tyres wear

Brian, this is ppedantic but as tyres wear there isn't a significant speedo effect. Reason is the the tyre tread is like a tank track. The steel in the tread stays a constant length and as the tyre tread goes a bit flat at teh footprint where it touches the road the tread surface circumference at this part compresses, thus creating a constant effective tread surface circumference at the road footprint, regardless of whether tyres are new or worn. Sorry, v pedantic! :-)

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Re: Testing the Speedo

The best place to test the speedo.s accuracy is a straight section of Motorway.

Using the Km posts and a stopwatch, keep a constant speed. The result should be pretty accurate and would probably match the GPS reading.

Fast speedos, legally up to 10%, are the reason for dawdlers hogging the overtaking lane, thinking, wrongly, that they are doing 70 when they are really doing about 63! Their speedo needs to read 77 for them to be doing 70.

They should pull over anyway.

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