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I don't want to cause thread drift here, but how do GPS speedometers give a true ground speed reading when the vehicle is going uphill or downhill?
Dave
vbalddave
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I went the 'cheap Chinese' route (because I hope that it will be a temporary fix while I get my main speedo to work with wonderful accuracy in spite of the cable currently being incompatible with the Type 9 5 spd box 🙂) .... my Defender has a spot-on accurate mechanical speedo (measured against various GPS and roadside speed warnings) ... so I tested it out by fitting it in the landy ... was surprisingly accurate and except for taking 30 secs to 'boot up' initially seems remarkably stable here in the NE Scotland.
Having an idea what revs correspond to what speeds as a temporary fallback for if you do get a drop-out might be reassuring.... though I guess in your case you can temporarily forgo the HUD in favour of the main speedo if that happens
I believe TBM has used one in his Morgan for some time, so maybe he can give us a 'long term test' report?
K Yes, a cable speedo _can_ be accurate, but it's that pesky 90 degree bend to get the cable out from the gearbox and the up the side of the transmission tunnel that I reckon causes the 'wavy needle' problem. Plus I also use a 14" steering wheel that partially obscures the speedo, so I figure a bit of extra info can't hurt. As you say, hopefully Phil will be along shortly to give us the benefit of his experience.
Tim H.
1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE
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I don't want to cause thread drift here, but how do GPS speedometers give a true ground speed reading when the vehicle is going uphill or downhill?
Dave 3 Dimensional trilateration using at least 4 intersecting spheres. https://electronics.howstuffworks.com/gadgets/travel/gps.htm
Tim H.
1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE
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Talk Morgan Guru
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I don't want to cause thread drift here, but how do GPS speedometers give a true ground speed reading when the vehicle is going uphill or downhill?
Dave Multiple satellite reception from a range of angles would ameliorate what would in any case be extremely low impact on horizontal speed errors. I guess a Eurofighter in vertical climb would present a problem although even then GPS altitude changes could give a speed reading. ETA: the fourth satellite requirement for minimum 3D location is needed for almanac data received from satellite. Most GPS receivers have 12 channel or more capability.
Last edited by RichardV6; 24/05/22 09:39 AM.
Richard
2018 Roadster 3.7
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2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone
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Thanks, Tim. I can genuinely say that having read the article, I'm not much better off (though I did understand the word "sphere", so that's something). I'm happy to take your assurance on this one!
Dave
vbalddave
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Thanks, Tim. I can genuinely say that having read the article, I'm not much better off (though I did understand the word "sphere", so that's something). I'm happy to take your assurance on this one!
Dave According to the operators of the GPS system, the accuracy offered by the satellite data is theoretically quite high, but it also depends on the implementation in the receiver:
How accurate is GPS for speed measurement?
As with positioning, the speed accuracy of GPS depends on many factors.
The government provides the GPS signal in space with a global average user range rate error (URRE) of ≤0.006 m/sec over any 3-second interval, with 95% probability.
This measure must be combined with other factors outside the government's control, including satellite geometry, signal blockage, atmospheric conditions, and receiver design features/quality, to calculate a particular receiver's speed accuracy.
Tim H.
1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE
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I was going to ask a similar question regarding the purchase of a GPS speedometer. Unfortunately, all the ones I came across, claimed to be “head up” display reflecting on the windscreen. I would prefer something simple that I could see.
Regards, Alistair Alistair ... I used HUD as a shortcut for 'box with numbers on in your line of sight' ... rather than meaning a proper windscreen projected jobbie .... plenty of simple directly read digital speedos out there cheap as chips. K
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Talk Morgan Sage
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This is my solution - I rarely look at the speedo as it's not in line of sight, although I do like it to work  Got a black plastic stick on disc on the dash, and then the satnav sticks to it with the 'suction pad'. Allows me to see GPS speed, the speed limit on the road and gives me directions. The one in the photo is now considered obsolete by TomTom so now have a larger, newer one, but it does not obscure the quad gauge or rev counter. ![[Linked Image]](https://www.tm-img.com/images/2022/05/24/SATNAV.md.jpg)
1972 4/4 4 seater, 1981 MGB GT
1984 Harley Davidson Electra Glide, 1990 Kawasaki ZX10
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I cannot find the email reference to the GPS speedo I bought. I do remember struggling to sift out the various versions so that I ended up with something simple. IIRC, I got mine from Car Builder Solutions. This one is the same / similar: https://www.carbuilder.com/uk/wireless-satellite-speedometerOn my 2014 4/4, I was able to make a simple mounting bracket from an old tupperware container to end up with a plastic angle bracket that is fixed to the top of the column shroud with velcro tape. On your earlier car, you may need a more complex mounting. No changes to the dashboard or to anything visible. You can just about see it in this picture of Mrs F taken at the DonMog grass auto test a couple of weeks ago. Taken with an iPhone which appears to have distorted the perspective at the close up part of the picture. Mrs F's perspective is fine - fortunately. ![[Linked Image]](https://tm-img.com/images/2022/05/24/JPEG-image-6.jpg)
Last edited by Paul F; 24/05/22 11:03 AM.
Paul
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The odd thing is that both my Morgan speedos are almost spot on, so I don't really need the GPS speedo, EXCEPT that on super cold mornings, the Plus 4 speedo mists up so bad that it is unreadable, so the GPS is a godsend.
DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4
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