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Which speedo to drive by?

  • GPS Based (Road Angel, TomTom etc.)

    Votes: 17 89.5%
  • Car Speedometer

    Votes: 2 10.5%
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Premium Member
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Discussion Starter #1
This is a little follow-on from Melvin's 'optimistic' thread about speed. How much should you trust the speed readout on your Tom Tom etc.?

When I'm doing 70mph, my NavMan says I'm doing 61, I need to do roughly 80-ish to get my GPS to say 70. Now I can't physically bring myself to drive through a speed camera at what my speedo says is 10mph too fast. Ordinarily I would say listening to the GPS is the sensible thing to do, but working with GPS in my daily work I know they are prone to being anything up to 30 meters out distance wise so why not a few mph out speed wise? Also if the GPS were to be out and I got a ticket, I wonder what kind of defence that would be in court "I thought my speedo was out, so went by GPS m'lud".

Finally, who does speedo calibrations???
 

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Premium Member
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Discussion Starter #3
Just did that, it says when I'm doing 70, i'm actually doing 71.7 or something :confused: But GPS says 60. Go figure.
 

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I still have kph speedo so use the gps, crude calc is KPH x 1.5 +10%, so 30mph = 49kph, 50mph = 82, 70mph= 115, gps and speedo match (Syrius GPS)
 

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ive done a few tests on this cos i found the same thing, although my speedo wasnt quite as far off as yours.

two different road angels in several different cars all gave the same reading. the speedos were all reading higher than the gps and the gps/speedo relationship was always the same. unfortunately its not conclusive evidence that the gps is accurate but i would imagine it would be too much of a co-incidence for them to be wrong by the same amount(if you follow that).

all speedos do read high by a certain amount afaik. if you look at proper road tests on cars there are quite often two speeds quoted, speedo mph and true mph.

my old supra was the furthest out of all, 100 on gps was 92 on speedo. i did trust the gps through cameras etc and never had a problem so my opinion is that the gps was accurate in this case.
 

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My Stagea reads slow by about 7% according to the GPS, whilst my Vectra reads 4-8 mph fast.

This can get frustrating, because I think people are driving slower than they are.

I generally use the GPS in the Stagea to help with the speed.
 

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516ATF and RWD......Nutter!!
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Went through Santa pod's speed trap at 110 on my speedo and it was only 100 by their speed.

Not sure even 30m is going to be a problem when it comes to doing 70mph.
 

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I've now had 2 navman satnavs as the first one got nicked, long story. Both of them read 3 MPH less at 70 MPH with my company car. I must get round to trying it in my skyline
 

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Administrator
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I've tried 2 different Road angels and 2 different Tom Tom's in my car and they always read lower than my speedo, i always go through the speed cameras of my road angel gps reading.

Its a known fact that speedo's are set to read a tad high from factory to lower the chance of people getting caught speeding.
 

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Seasoned Member
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dissagree with ya bruce, if that was the case then all the mpg, 0-60 yop speed etc figures would be wrong...

my sat nav (tom tom) reads 3 mph slower than my skyline...merc vito...fiesta..new clio...my snooper reads exactly the same as my speedo according to the gatso's...so i conclude the gps signal must have a period of delay from satellite to satellite......;)
 

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I drove a constant speed through one of those measured miles, and working out the timing, the sat nav was spot on (TT Go710). Speedo was reading aprox, 6mph higher than sat nav at 70mph.

I now believe everything my satnav tells me, lol.
 

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As long as you have a good sat connection and are doing a steady speed the Road Angels et al are extremely accurate, more so than any speedo on a 10 year + old car. The sat devices update roughly once a second, so they are not brilliant at reading ever changing speeds.

My GTR is about 10% out in reading speed over its entire range (it reads about 77 when I'm really doing only 70, and about 33 when I'm actually doing 30). This has been checked, so I always go by my Road Angel. Never trust your speedo, especially on an import that has had any kph to mph conversion done on it (or has been removed for any reason), and especially on an oldish car. If you think your speedo is accurate then you are actually just one of the lucky ones.

An easy way to check .... find a road that has one of those new radar speed displays, the ones that flash the speed that you are doing, usually on a stretch of 30mph road. They are surprisingly accurate if you are the only car on the road. I've gone through quite a few and every single time it matches exactly what the Road Angel says and not my speedo.
 

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Seasoned Member
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the simple answer is to get on the motorway have the police chasing you and you hold a steady ton on yr speedo and we can all watch on "police camera action" then we will know if its accurate....oh and by the way when yr being chased take it 150mph on yr speedo so we can see what the cops make it at higher speed mate
 

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the simple answer is to get on the motorway have the police chasing you and you hold a steady ton on yr speedo and we can all watch on "police camera action" then we will know if its accurate....oh and by the way when yr being chased take it 150mph on yr speedo so we can see what the cops make it at higher speed mate
ill try that first thing tommorrow mate





cue thread of my skyline for sale :D
 

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BEDFORDSHIRE......
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My Speedo says MPH but when I pull away it get a 0-60 in 2.1 secs???:D it is acctually KPH. Dont know why they did this? PUT A STICKER OVER THE KPH? Anyway I went out over the weekend with the sat nav onboard and at 30 MPH (on the nav) ther speedo was at 50. after the nav was showing 50MPH and the speedo was at 80. I got it up to 70 on the nav...speedo was 120. and at 100MPH the speedo showed a tad over 180. I then gave it up as the speedo only goes up to 180.

TOM TOM 500.
 

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I have a talex lite GPS camera thingy, I was doing 74mph according to it when it said " Warning, possible mobile camera site ahead, limit 60mph....attention reduce speed, attention reduce speed....." etc.....a week later i got a letter from the cops, 74mph in a 60 mile an hour limit. DOH !!! So mines pretty much on the mark, even though my speedo was saying I was doing 90+ then.
 

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Have to go with the Satnav as being accurate (usually!), speedo about 76 roadangel 70, speedo 100 (private road) roadangel 92, however, stopped last week, speedo 0, roadangel 8mph???????:uhoh: Definitely a delay there somewhere, so I trust the angel when moving and speedo/senses when stopped :D
 
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