Faults in Speedometers and how to fix them.

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1400 posts

Uber Geek

#3174983 23-Dec-2023 11:45

@scubadoo

20 year old Garmin (I have hand held one) used old spec chips, newer GPS chips in portable devices (like my Garmin wrist watches) have better sensitivity and faster cold start. I used both Garmn and TomTom devices simultaneously on a stretch of SH1 at Cruise Controlled Speeds to capture discrepancies with Speedometers in various Cars - Mazda, Toyota, Nissan, Honda and to measure one suburban route as a reference track for known distance.

I also recorded CAN bus speed signals and can run speedometers on the bench at known speed to check various functionality.

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RUKI

1400 posts

Uber Geek

#3174987 23-Dec-2023 11:56

FYI: for those annoyed with speedometer underreporting speed in older cars where speed is not transmitted over CAN bus but as a pulse signal (e.g. Lexus IS250/350/ISF) 2006-2010 there are of the shelf devices for your DIY install which can adjust that.
I bought 3 from different vendors and tested with IS350 2008. One was junk, returned to vendor, two actually worked. Still have them both if anyone wish to play.

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SomeoneSomewhere

1754 posts

Uber Geek

Lifetime subscriber

#3174988 23-Dec-2023 11:57

SCUBADOO:

alasta:

How accurate is GPS as a speed measure in a car?

As a runner, I have my sports watch set to show my speed over a rolling average kilometre because the instantaneous speed measure tends to fluctuate and lag a bit.

Even my now 20 year old hand held Garmin GPS claimed in the specs 0.328 ft./sec. when in a steady state straight line.

That equates to 0.36 km/h.

The current GPS satellite network must have well and truly improved on that over the years.

I think you will find that's the limit of precision, not accuracy. It's almost exactly 0.1 meter per second - likely some step in the chain rounds the value to that many decimal places.

This app displays a whole bunch of data about how your cellphone GPS is working under the hood, including estimated speed and position accuracy. It's typically several km/h, though one would expect it to over-read and under-read roughly equally.

I think averaging it over a longer time period would be necessary to get very good results, or use differential-GPS gear that can deliver much higher accuracy using a separate ground station.

EDIT: It goes without saying that speed accuracy will vary heavily depending on view of the sky, current satellite positions, possibly weather, and other factors, just as position accuracy does. A GPS value can't really be used as a replacement for a wheel-based speedo as the GPS will obviously fail in tunnels. That said, I would think that GPS-disciplined local speedometers could be an option, where it uses good GPS signals to correct the reading from the wheel RPM sensors, compensating for e.g. tyre wear and inflation.

SomeoneSomewhere

1754 posts

Uber Geek

Lifetime subscriber

#3174989 23-Dec-2023 11:59

shk292:

It's a minor irritation that Google maps on Android Auto or Carplay doesn't show GPS speed - I guess for the same reason as the inbuilt nav systems don't

Presumably if you displayed speed from the OBD port, it would be true not "fudged" to over-read

Presumably Nissan Leaf speedos all over-read badly because they always seem to drift along at 85-90 on a clear motorway

I think that may also be poor battery range. Leafs are the oldest and lowest capacity EVs on the market, along with faster battery degradation than other models. Many have rather low range, and going 10-15km/h slower can result in fairly significant drag/power/fuel savings.

Goosey

2775 posts

Uber Geek

Subscriber

#3174991 23-Dec-2023 12:34

I use a seperate GPS for time and speed.

I find it’s only 3km variance when heading south (or somewhere the gps on the windscreen can’t quite see the sky), and 2km variance when heading north

if I’ve got under inflated tyres it increases.

I also find those road side digital speed check signs to be highly accurate vs the gps displayed speed.

2020 mazda cx5

blackjack17

1702 posts

Uber Geek

#3175021 23-Dec-2023 14:01

shk292:

It's a minor irritation that Google maps on Android Auto or Carplay doesn't show GPS speed - I guess for the same reason as the inbuilt nav systems don't

Presumably if you displayed speed from the OBD port, it would be true not "fudged" to over-read

Presumably Nissan Leaf speedos all over-read badly because they always seem to drift along at 85-90 on a clear motorway

I noticed on a road trip in the States back in 2019 some states had the GPS speed showing on google maps on the phone while others didn't.

Could it be a legal thing?

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RUKI

1400 posts

Uber Geek

#3175031 23-Dec-2023 14:42

@shk292 - in many modern cars, e.g. 2019+ Toyota, Mazda, Honda, Nissan - you won't get speed signal from the OBD port as the new cars have gateways to disallow DIY people to plug rubbish scanners and aftermarket dongles into OBD ports.

Leafs - you are right in your observations about them being slow in the 100 limit zone.
I have recorded the whole CAN bus trace from ZE1 and it is now driving at speed on the bench. Was not from the OBD port though..It helps to check faulty clusters like the recent case with the frozen ODO.

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Eva888

2351 posts

Uber Geek

Lifetime subscriber

#3175061 23-Dec-2023 18:13

@RUKI thanks for the explanation. I thought there was something wrong with my speedo reading more than the GPS. You sound very talented and I wish you were in Wellington. Do you recommend anyone here for Lexus and Aqua?

RUKI

1400 posts

Uber Geek

#3175065 23-Dec-2023 19:36

@Eva888 I help NZ-wide and AUS 7x365, turnaround 24hours. E.g. parcel arrives today, couriered back next morning. For Aqua and many other models I have an arrangement when your car will be off the road (e.g. without a cluster) only for an hour regardless if you are in Wellington or Sydney.

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tim0001

261 posts

Ultimate Geek

#3175071 23-Dec-2023 20:57

@RUKI . Just curious whether you see many instrument clusters with issues caused by ageing electrolytic capacitors?

I’ve got a 1993 (vintage?) Toyota that had a few leaky caps in the instrument cluster and ECU. The ECU had a couple of caps that leaked through the conformal coating and started corroding the PCB traces. Interestingly, the measured performance of these old caps was pretty good considering. Their capacitance was still in spec and their ESR was only about twice that of the replacements.

tecnam2003

64 posts

Master Geek
Inactive user

#3175074 23-Dec-2023 21:50

But surely this inaccuracy will also be reflected in the odometer and for Diesel vehicles, an impact on the RUC.

I have a 2 year old diesel campervan (VW based) that has a wheel mounted ODOMETER. The front plate of the odometer is engraved with the number of revolutions per KM.

This relates then to the tyre size and when I compared the wheel meter with the vehicle instrument odometer there was a discrepancy in favour of the Goverment's RUC. The wheel meter is not correct for the tyre size but when trying to chase down an answer I was told it with a margin of acceptable error.

Just saying

RUKI

1400 posts

Uber Geek

#3175091 23-Dec-2023 22:54

@tecnam2003 speed display has inaccuracy as explained, but kms travelled display are actually accurate at stock tires. As tested at my reference track in various cars >2012.

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RUKI

1400 posts

Uber Geek

#3175092 23-Dec-2023 23:09

@tim0001 re: faulty capacitors in clusters - have not seen any related failures yet. But I understand the issue with capacitors - fixed few PS, Plazma TV, LCD TV before and have heaps of various high quality caps from dismantled Japanese electronics.
What I noticed however in absolutely all Honda Vezel / FIT / Grace speedometers 2014+ : distinctive sign of power elements excessive heat - i.e. brown colorization around Mosfets and Voltage regulators. But no visible cracks which you'd find in overheated power supplies around heavy elements.

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BlakJak

1245 posts

Uber Geek

Trusted

#3175493 25-Dec-2023 21:42

I've understood - and verified through various means over the years - that the variance is more likely a percentage variance, the max generally allowed is 10%, it's usually ~7-8%, digital speedo's and ubiquitous GPS comparisons have made this more obvious. The delta is not linear, but my Prius is usually about 5km/h out at 50km/h and 8km/h out at 100km/h.

Knowing that and understanding the significance of it, i'm fine with it.

As to whether it causes other people to drive too slowly - well... possible. But i'm not sure we can do anything about that.

No signature to see here, move along...

RUKI

1400 posts

Uber Geek

#3175902 27-Dec-2023 23:29

JDM speedo in Prius 2010-2014 including PHEV version can be reprogrammed to English. Done it many times. Those speedometers have 2 transformers inside. Wth age - vibration damages internal PCB connections, it goes black and those are unrepairable. Replacement only. I have one 2014 hybrid spare.
One car dealer once dismantled it just to remove dust from under the glass. Quite a lot to dismantling for that matter. No clue how that much dust have gotten inside the cluster. Saw it only once like that.

To help people to take it out for inspections / repairs / conversions - I made a video how-to dismantle it few years ago. Steps 1-26. Step 27 is for something else.

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