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Speedometer accuracy
- ronk
- Lifer
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- Location: Durham
Speedometer accuracy
I’ve been looking at the accuracy of my Speedo accuracy after raising the tyre profile by 5%
I used an iPhone app and at 30/40/50 mph the digital read out and thd analogue readout coincide identically
Can the iPhone app be trusted ?
I used an iPhone app and at 30/40/50 mph the digital read out and thd analogue readout coincide identically
Can the iPhone app be trusted ?
You don't stop playing when you get old - You get old when you stop playing!
So I bought a 35is with all the toys to play with.
So I bought a 35is with all the toys to play with.
- Ed Doe
- Senior Member
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- Location: Ashbourne or Frimley
Speedometer accuracy
What app is it?
What GPS polling rate does your device offer? Normally for phones this is 1hz, which is fairly low, and is why people use a remote GPS receiver with a higher polling rate to get more accurate data overlay for track days. Same stands to reason for obtaining accurate speed data on the road though
Generally also worth noting that all manufacturers will deliberately make their car speedos over-read, to account for variances in tyre size (the difference between 8mm of tread on a new tyre and 1.5mm of tread on a tyre that needs replacing equates to a significant change in diameter, and therefore speed), or indeed people sticking larger profile tyres on their cars. You can imagine the furore if a person got caught speeding but their speedo said they weren't!
What GPS polling rate does your device offer? Normally for phones this is 1hz, which is fairly low, and is why people use a remote GPS receiver with a higher polling rate to get more accurate data overlay for track days. Same stands to reason for obtaining accurate speed data on the road though
Generally also worth noting that all manufacturers will deliberately make their car speedos over-read, to account for variances in tyre size (the difference between 8mm of tread on a new tyre and 1.5mm of tread on a tyre that needs replacing equates to a significant change in diameter, and therefore speed), or indeed people sticking larger profile tyres on their cars. You can imagine the furore if a person got caught speeding but their speedo said they weren't!
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- B21
- Lifer
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Speedometer accuracy
Yup the iPhone apps are perfectly ok…if you are monitoring the speed whilst in a steady state ie holding a speed the refresh rate is essentially irrelevant to this requirement…
I’ve compared a Dragy which has a more sophisticated GPS engine and at steady speed there was no practical difference..FWIW generally I’ve found across many cars that going up one click on tyre sizes makes most speedo reading on or just below the true speed..with one click on tyre sizes I’ve never seen the speedo under read..but..it means for sure that you no longer have a 10% ish leeway..
Higher refresh rates are relevant with fast accelerating objects..
I’ve compared a Dragy which has a more sophisticated GPS engine and at steady speed there was no practical difference..FWIW generally I’ve found across many cars that going up one click on tyre sizes makes most speedo reading on or just below the true speed..with one click on tyre sizes I’ve never seen the speedo under read..but..it means for sure that you no longer have a 10% ish leeway..
Higher refresh rates are relevant with fast accelerating objects..
We choose to go to on with this endeavour at this time and do the other things, not because they are easy, but because they are hard…
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A very modified Atacama Yellow 35is
- ronk
- Lifer
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Speedometer accuracy
I will have a look into the app details asap !
The Speedo and iPhone app coincide very well now - but as you say, no margin for error!
The Speedo and iPhone app coincide very well now - but as you say, no margin for error!
You don't stop playing when you get old - You get old when you stop playing!
So I bought a 35is with all the toys to play with.
So I bought a 35is with all the toys to play with.
- Ed Doe
- Senior Member
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- Location: Ashbourne or Frimley
Speedometer accuracy
And such trivialities as incline/declines which will affect the relative speed in the X-direction, which is what GPS measures....
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Silver Grey '06 3.0si Coupe - SOLD
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- ronk
- Lifer
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Speedometer accuracy
I can’t find any tech details in the app - it was just a free download from the App Store
I only needed it to attempt to verify that my speedo‘ wasn’t way out of accuracy and as I said, at 30/40/50 the app and Speedo coincide perfectly.
(https://www.timosstudios.com/)
I only needed it to attempt to verify that my speedo‘ wasn’t way out of accuracy and as I said, at 30/40/50 the app and Speedo coincide perfectly.
(https://www.timosstudios.com/)
You don't stop playing when you get old - You get old when you stop playing!
So I bought a 35is with all the toys to play with.
So I bought a 35is with all the toys to play with.
- B21
- Lifer
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- Joined: Tue Oct 05, 2021 7:15 pm
- Location: In my Banana Yellow Space Shuttle...somewhere over Southern Caledonia
Speedometer accuracy
All GNSS systems are approx only 1/3 as accurate in the vertical domain due to geometry unless they use baro assist or local area augmentation..for the purposes of Ron I think his app on iphone is more than good enough..in addition (I'm not sure how they do it) the later phones integate positional calculations from more than just the US GPS system..I suspect the Russian GLONASS system isn't being used a lot..
In aviation we can'r really trust GPS for vertical navigation unless it has some form of augmentation..in part because of geometry and also GPS uses an approximation model of the earth's surfaces based on a WGS84 Geodescic model..which as its name implies is a model of the Earth's surface and imperfect enough not to be relied on for safety of life applications..in isolation..
We choose to go to on with this endeavour at this time and do the other things, not because they are easy, but because they are hard…
A very modified Atacama Yellow 35is
A very modified Atacama Yellow 35is
- ronk
- Lifer
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- Joined: Tue Feb 24, 2009 11:50 am
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Speedometer accuracy
That sounds like witchcraft!
You don't stop playing when you get old - You get old when you stop playing!
So I bought a 35is with all the toys to play with.
So I bought a 35is with all the toys to play with.
- B21
- Lifer
- Posts: 5466
- Joined: Tue Oct 05, 2021 7:15 pm
- Location: In my Banana Yellow Space Shuttle...somewhere over Southern Caledonia
Speedometer accuracy
Not really..but it was one of the few examples of where the practice eclipsed the theory in terms of actual accuracy vs predicted accuracy..so much so that for many years the US added 'selective availablity' to the civilian version of the signals cause they were almost as accurate as the mil version that was being used for weapons etc..they deliberately added 'dithering' to make the fix you saw on the ground where you were appear to walk around you a bit..President Clinton revoked it..now ith other GNSS like Galileo its irrelevant.
We choose to go to on with this endeavour at this time and do the other things, not because they are easy, but because they are hard…
A very modified Atacama Yellow 35is
A very modified Atacama Yellow 35is
- sars
- Lifer
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Speedometer accuracy
As for how accurate speedometer should be, from regulation No. 39, Type test approval of the Speedometer.
5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified
in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1)
and the true speed (V2).
0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h
5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified
in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1)
and the true speed (V2).
0 ≤ (V1 – V2) ≤ 0,1 V2 + 4 km/h
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- Mr Tidy
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Speedometer accuracy
I went up a profile from 40 aspect ratio to 45 on my rear tyres and according to my Garmin it's now doing a real 69 when reading 70 on the speedo.
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Current - Silver Grey MC, Imola Red heated Nappa & carbon trim. Aeros, H & R Coil-overs, 224s, OE Strut brace, Nav, cup-holders, DSP Hi-Fi, pdc, cruise, MFSW, no CDV! E90 330i daily
Gone - Montego Blue
Gone - Ruby Black