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Wales
by Joske Vermeule, September 1
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Forums39
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Jun 14th, 2026
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Joined: Mar 2011
Posts: 831 Likes: 23
Talk Morgan Regular
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Talk Morgan Regular
Joined: Mar 2011
Posts: 831 Likes: 23 |
Morgan now it's on 15 rather than 14 inch rims and 205/60 reads 5% over. My Mercedes is accurate to 1MPH at 70, 1.5% over. (Both according to Waze and TomTom)
1980 +8 Blue
And a few others
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Stephen
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Joined: Dec 2007
Posts: 536
Talk Morgan Regular
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OP
Talk Morgan Regular
Joined: Dec 2007
Posts: 536 |
Stefan, you may be right. If owners get "better" fuel consumption because of the optimistic speedo they are more likely to achieve the claimed figures of manufacturers.
Cheers
Philip
1985 Plus 8 four seater
Malone F1000 - track car
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Joined: Feb 2016
Posts: 8,684 Likes: 414
Needs to Get Out More!
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Needs to Get Out More!
Joined: Feb 2016
Posts: 8,684 Likes: 414 |
Richard, I understand about the law but why such big over-reads. Speedos could certainly be more accurate, witness the 2cv. Also why stop in-built GPS devices showing true speed?
One of the reasons it's getting to me at the moment is that living in France I do a lot of driving on N & D roads that are most of the time limited to 80km/h (though some departments are 90km/h). Lots of drivers understandably are wary of all the speed cameras and so drive along at an indicated speed of just under 80km/h which, is often a true speed of a little over 70km/h. I'm all for safety but such low speeds on good N & A roads are crazy and in fact some times causes people to overtake dangerously because they find the low speed unacceptable.
Cheers
I agree Phillip but cannot explain why such large over reads are allowed. I've been aware for some time of the 0 to 10% over read allowance and clearly they shouldn't under read, but the extra 10km/h (6.25mph) I only discovered recently. Regarding GPS speed displays, they do seem to have a variety of damping and averaging algorithms. I've noticed that commercial devices incorporating a GPS receiver do seem to display a higher speed figure than a raw GPS app on your phone for example. Could it be the manufacturers of such devices including those fitted to cars have a vested interest in ensuring you never get an under read displayed ![[Linked Image]](https://www.series2club.co.uk/new_forum/Smileys/wittsends/nixweiss.gif)
Richard
2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone
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Joined: Jun 2018
Posts: 641 Likes: 15
Talk Morgan Regular
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Talk Morgan Regular
Joined: Jun 2018
Posts: 641 Likes: 15 |
I read today, that optimists on average live longer than pessimists, so perhaps it is good to have more optimistic things round you.
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Joined: Jul 2007
Posts: 27,822 Likes: 750
Member of the Inner Circle
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Member of the Inner Circle
Joined: Jul 2007
Posts: 27,822 Likes: 750 |
That depends. I'm so depressed; brain the size of a planet.  GPS measures effectively in a series of straight lines as I understand it. Speedo measure distance travelled. Rapid acceleration, braking & or bendy roads can throw my GPS speedo but it is bang on vs those flashing speed signs when the speed is constant. I rely more on the revs as 2k revs = 20 in second , 30 in 3rd, 40 in 4th & 68 in top
JohnV6
2022 CX Plus Four
2025 Renault Scenic Alpine etech "Yvette"
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Joined: Nov 2018
Posts: 1,065 Likes: 23
Has a lot to Say!
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Has a lot to Say!
Joined: Nov 2018
Posts: 1,065 Likes: 23 |
In some European countries you can ask the factory that your car must be so fast as the datasheed says...
And it all was fine until my speedo told me: 180+ with hood down. Wow.
Back home from the summer fast road trip the speedo shows 180+ and the gps high the ton.
Hmm. Only knowing the speedo the world was right. Now: hmm, she could be a little bit faster ;-)
René
2018 plus4 (sports green)
with a homemade really shiny red tinted dash…
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Joined: Jan 2019
Posts: 938 Likes: 46
Talk Morgan Regular
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Talk Morgan Regular
Joined: Jan 2019
Posts: 938 Likes: 46 |
Thank you John for bringing in the gps measuring principle.
GPS itself is not hat accurate. It can have a 1.5m inaccuracy with a standard receiver.
It than measures straight lines in a frequency of 1 Hz.
So, when you travel a straight motorway and the algorithm calculates a couple of iterations before displaying speed, it is quite good.
On curvy roads...
So please do not overestimate the accuracy of gps speed.
On the other hand speedometers have improved a lot over the latest years;-) So it is often not possible to calculate an extra 10% as the Mercedes example shows.
Also the rev calculation depends on pressure as well as tyre wear.
If you travel exacly to the point - you should be on the safe side;-)
1973 2CV AK400 Ralley
1985 plus 4 four seater
1992 Peraves ecomobil
bicycles (pedersen to recumbent)
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Joined: Feb 2016
Posts: 2,033 Likes: 23
Talk Morgan Expert
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Talk Morgan Expert
Joined: Feb 2016
Posts: 2,033 Likes: 23 |
Reminds me I really need to recalibrate both of mine, my Aero over-reads at least 10% maybe more, so its probably done at least 3k less than the clock shows! 
Aero 8 GTN #11
"What we do in life.. echoes in eternity."
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Joined: Nov 2013
Posts: 1,947
Talk Morgan Enthusiast
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Talk Morgan Enthusiast
Joined: Nov 2013
Posts: 1,947 |
You can have a fairly accurate odometer with very optimistic speedometer.
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