Speedometer accuracy

[ronk]

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 ?

[Ed Doe]

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!

[B21]

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..

[ronk]

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!

[Ed Doe]

Higher refresh rates are relevant with fast accelerating objects..

And such trivialities as incline/declines which will affect the relative speed in the X-direction, which is what GPS measures....

[ronk]

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/)

IMG_6691.jpeg (23.85 KiB) Viewed 281 times

[B21]

Higher refresh rates are relevant with fast accelerating objects..

And such trivialities as incline/declines which will affect the relative speed in the X-direction, which is what GPS measures....

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..

[ronk]

That sounds like witchcraft!

[B21]

That sounds like witchcraft!

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.

[sars]

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

[Mr Tidy]

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.