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Understanding GPS activated digital speedo

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MGB567 Avatar
MGB567 Gold Member Barrie Braxton
Sunshine Coast, Queensland, Australia   AUS
1966 MG MGB MkI "Money Guzzler"
1979 MG MGB GT V8 Conversion "Darkside"
My speedo below is a Smiths Telemetrix triggered by a GPS signal receptor mounted on the dashtop. For my own reasons I want to have a digital version in between it and the tacho but will having two GPS signal receptors cause interference?



Convertible: CKD 11/66 first registered 8/5/67. Owned since 3/77. 90% original sheet metal. 18GB +40 balanced with almost all new internals. Peter Burgess big valve fast road head. Piper 285. Fidanza FW. Basil's followers and pushrods. TR7clutch. TT exhaust. ARP everywhere. 123 ign. Needham 4synchro c/r box.. Stock rebuilt/replaced suspension. Superpro bushes. New brakes all round including all pipes in SS flex. Interior redone. CAMS approved roll bar and side bars. Lots more. Hybrid of o/e and show/fast road car. Not for sale - it's my toy!

GT: UK car built/sold December '78. Stripped back to bare shell (with extensive bodywork to come). Powered by 'worked' Rover 5 litre V8 (ex TVR Chimera) with efi. T5 box. FC IFS. CCE rear attached to Salisbury axle with Quaife. And a whole lot more to yet to come. Stealth is the word.

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ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
No



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

JohnyMc Avatar
JohnyMc Gold Member John McFarlane
Mossman, Queensland, Australia   AUS
I would think you should speak with the manufacturer.


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melbaver Avatar
melbaver Gold Member Chris Howells
Broadwater NSW, Australia   AUS
1968 MG MGB "Moneypit"
1996 Jeep Cherokee "Shopping Trolley"
No. Exactly like having a Tom Tom and your smartphone navigating at the same time. In fact you'll probably do that with the new speedo to get an idea of calibration, and that would involve three discrete non-interferential GPS signals.



Chris Howells

1968 MGB Purchased already dis-assembled but which is largely back together so I'm a lot less ignorant.

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MGB567 Avatar
MGB567 Gold Member Barrie Braxton
Sunshine Coast, Queensland, Australia   AUS
1966 MG MGB MkI "Money Guzzler"
1979 MG MGB GT V8 Conversion "Darkside"
Thanks. My DD is blue tooth enabled and paired with my I4 (I know but there's a reason). Often enough driving past another car my "voice in" activates. I thought something similsr might happen with GPS locators. Good to know I'm good to go! (well when it's on the road smiling smiley )



Convertible: CKD 11/66 first registered 8/5/67. Owned since 3/77. 90% original sheet metal. 18GB +40 balanced with almost all new internals. Peter Burgess big valve fast road head. Piper 285. Fidanza FW. Basil's followers and pushrods. TR7clutch. TT exhaust. ARP everywhere. 123 ign. Needham 4synchro c/r box.. Stock rebuilt/replaced suspension. Superpro bushes. New brakes all round including all pipes in SS flex. Interior redone. CAMS approved roll bar and side bars. Lots more. Hybrid of o/e and show/fast road car. Not for sale - it's my toy!

GT: UK car built/sold December '78. Stripped back to bare shell (with extensive bodywork to come). Powered by 'worked' Rover 5 litre V8 (ex TVR Chimera) with efi. T5 box. FC IFS. CCE rear attached to Salisbury axle with Quaife. And a whole lot more to yet to come. Stealth is the word.

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
I run a VDO GPS speedo and use my Garmin for a digital. So two GPS receivers within inches of each other and no problem.



Cogito ergo sum periculoso

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Oh, I think there are speedo programs for phones you could use to test if you had any concerns.



Cogito ergo sum periculoso

Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
We run GPS systems with multiple antennas at work and it's not a problem (it allows for accurate slip calculations and localisation etc)

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MGUK Paul Wiley
Watton, Norfolk, UK   GBR
A small point regarding GPS devices. Most of them come calibrated to use a sort of universal model of the world. BUT the world isn't truly round. The universal setting works reasonably but in the UK I found with a simple Garmin hand held GPS it was reading about 100 metres of when checked against the local paper maps (known as Ordnance Survey) The paper maps are very accurate but are based on an Ordnance Survey model. Fortunately the simple Garmin could be set to use this model and many others if required.

Now in Australia there is almost certainly a local model and the GPS would be more accurate regarding location if it is used.

There is a story that in one of the Desert Wars a few years ago, an Australian patrol was calling on support from American artillery. The troops gave their and the target's location using their GPS devices. The Americans set their guns and nearly blew up the Australians. This was because no-one had coordinated the GPS devices used by each army to use the same model in Kuwait and Iraq!

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
On the Garmin, I had an old one that compared exactly with a stop watch and mile markers as well as the big radar speed signs. Then after an "upgrade" it consistently read one MPH higher. My current one is the same. I have not had enough miles on my VDO GPS speedo to test. The things I don't like about it are in the sunlight, you can't see the odometer and it does take a block or so to sync. I had one of those cheap HUD speedos in my GT for a while and it read right with the Garmin. It was a lot more flaky though.

I suspect this is from the Federal regulations that a speedo can be as much as 10% +/- 2 MPH off as long at it does not read low. This I got from BMW when they were defending their worthless speedo in the Mini. It was more than 10% off. That said, I found out the hard way my GTI speedo is 4 MPH low at 77. ( The speedo was more accurate than the fuel gauge though)



Cogito ergo sum periculoso

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Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
In reply to # 3695635 by MGUK A small point regarding GPS devices. Most of them come calibrated to use a sort of universal model of the world. BUT the world isn't truly round. The universal setting works reasonably but in the UK I found with a simple Garmin hand held GPS it was reading about 100 metres of when checked against the local paper maps (known as Ordnance Survey) The paper maps are very accurate but are based on an Ordnance Survey model. Fortunately the simple Garmin could be set to use this model and many others if required.

Now in Australia there is almost certainly a local model and the GPS would be more accurate regarding location if it is used.

There is a story that in one of the Desert Wars a few years ago, an Australian patrol was calling on support from American artillery. The troops gave their and the target's location using their GPS devices. The Americans set their guns and nearly blew up the Australians. This was because no-one had coordinated the GPS devices used by each army to use the same model in Kuwait and Iraq!

Besides the world model being suspect, GPS is fairly ropy as well. It was designed to drop nuclear bombs on targets and a few hundred meters is close enough. GPS on a good day will give you about 30 metres accuracy which is good enough for a sat nav (you use other methods to correct the location) but its still fairly poor. DGPS uses a local transmitter to generate a correction signal which can give 2cm accuracy however the range of the transmitters is limited to about 5Km so you then need to subscribe to a correction service however these have issues as well.

Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
In reply to # 3695637 by tvrgeek On the Garmin, I had an old one that compared exactly with a stop watch and mile markers as well as the big radar speed signs. Then after an "upgrade" it consistently read one MPH higher. My current one is the same. I have not had enough miles on my VDO GPS speedo to test. The things I don't like about it are in the sunlight, you can't see the odometer and it does take a block or so to sync. I had one of those cheap HUD speedos in my GT for a while and it read right with the Garmin. It was a lot more flaky though.

I suspect this is from the Federal regulations that a speedo can be as much as 10% +/- 2 MPH off as long at it does not read low. This I got from BMW when they were defending their worthless speedo in the Mini. It was more than 10% off. That said, I found out the hard way my GTI speedo is 4 MPH low at 77. ( The speedo was more accurate than the fuel gauge though)

There's a tolerance applied to speedos as you said and it varies according to the country. The indicated speed is usually the maximum the car is going, usually you're going slower and the speedo is over reading. The only exceptions are the police ones which were usually specially calibrated for a lot closer tolerance. The reason for the tolerances are to allow for tyre wear, tyre pressure, different tyre manufacturers making the same "size" tyre to different dimensions, speedo drive gearing, allowing commonality of parts across a range of cars etc. Some of these issues are still valid, some like drive gear ratios disappear when you use things like CAN since a micro processor can manage a range of different divisions etc a lot easier that you can change drive ratios.

Wray Avatar
Wray Gold Member Wray Lemke
., SC, USA   USA
Some of the issues with non-marine GPS's relate to the view of the sky the GPS sees and the number of satellites in view of the antenna. The more birds it can see the faster the update. In the early days, before SA was turned off, the fix could be pretty far off, up to 300'. Speed readings were pretty consistent, since the error generated by "clock dither" was positional. With SA (selective availability) turned off the position fix came down to within a few meters. If your GPS doesn't agree with other GPS's then there is more likely a firmware problem. If you want sub-meter accuracy then you have to go to a differential system, or a dual freq machine, which isn't practical for most auto applications.

We've found some disparity among vehicle non-GPS odometers vs GPS. Some are spot on, others can be of most of a tenth of a mile off. This could be the result of a new set of tires that weren't the same size as the OE tires, etc. The GPS usually gives the best measurement compared to a speedo on a measured mile. You would think that modern cars, with their electronic speedos would be right on, that's not always the case.

ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Civilian GPS accuracy is 4meters 95% of the time 3meters depending on Satellites and equipment.
Under 5 meters 99% of the time in many areas.
The limits changed in the last decade for civilian accuracy.
Military is even far more accurate.
Chart below pulled from the NTSB performance analys report
Your cell phone GPS is accurate in good acquisition to 5 meters
The next gen Chip is going into phones that will allow 1 ft(30cm) accuracy

Global Positioning System (GPS) Standard Positioning Service (SPS) Performance Analysis Report - Nstb.tc.faa.gov


In reply to # 3695651 by Chalky
In reply to # 3695635 by MGUK A small point regarding GPS devices. Most of them come calibrated to use a sort of universal model of the world. BUT the world isn't truly round. The universal setting works reasonably but in the UK I found with a simple Garmin hand held GPS it was reading about 100 metres of when checked against the local paper maps (known as Ordnance Survey) The paper maps are very accurate but are based on an Ordnance Survey model. Fortunately the simple Garmin could be set to use this model and many others if required.

Now in Australia there is almost certainly a local model and the GPS would be more accurate regarding location if it is used.

There is a story that in one of the Desert Wars a few years ago, an Australian patrol was calling on support from American artillery. The troops gave their and the target's location using their GPS devices. The Americans set their guns and nearly blew up the Australians. This was because no-one had coordinated the GPS devices used by each army to use the same model in Kuwait and Iraq!

Besides the world model being suspect, GPS is fairly ropy as well. It was designed to drop nuclear bombs on targets and a few hundred meters is close enough. GPS on a good day will give you about 30 metres accuracy which is good enough for a sat nav (you use other methods to correct the location) but its still fairly poor. DGPS uses a local transmitter to generate a correction signal which can give 2cm accuracy however the range of the transmitters is limited to about 5Km so you then need to subscribe to a correction service however these have issues as well.



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972


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Chalky Avatar
Chalky David White
Coventry, Warwickshire, UK   GBR
A lot depends on where you're using it. In a big open area then you can see multiple satellites and accuracy is pretty good (even better with an IMU and an extended Kalman filter). We see lots of issues with tree cover (signal attenuation) and also with tall buildings (multi-path) causing the gps to jump round. A lot depends on the quality of the antennas/chipset/software.

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