ECU speed or GPS which is correct

[Guest001]

Was whistling along on Cruise yesterday with ECU deadreckoning readout on 100kph Thought all good Went past a speed camera and nothing flashed(hopefully)
When I got home i checked my dashcam readouts and OH DEAR both read 103 at the time I passed the camera.

Now I sit and chew my nails for 3 weeks and see which is right.

Any ideas and what is the tolerance. Was on the Bruce about 3 k south of the Burpengary fixed camera

No Notice but i didnt worry as was sure I wasnt speeding. Maybe I was and ECU is incorrect.

[tigger73]

Are you reading ECU speed from OBD?

This will still depend on what wheel/tyre size you’re running as I’m pretty sure it’s calculated off engine speed and gear.

The most accurate is to get a GPS speed and compare this to ECU or Speedo reading so you know how much out these are.


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[Guest001]

Are you reading ECU speed from OBD?

This will still depend on what wheel/tyre size you’re running as I’m pretty sure it’s calculated off engine speed and gear.

The most accurate is to get a GPS speed and compare this to ECU or Speedo reading so you know how much out these are.


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No I am using the secret menu on my RNS510 You get several options one being navigation. When you go into that there are several more options, one being dead reckoning and the first line is supposedly your true actual speed. It seems it is 3kph slower than the GPS readings on my dashcams.
Cant rely on speedo which at 100kph is 6kph out with GPS. Have tried several times to correct it using VCDS but it seems Aus vehicles wont do it.

[tigger73]

No I am using the secret menu on my RNS510 You get several options one being navigation. When you go into that there are several more options, one being dead reckoning and the first line is supposedly your true actual speed. It seems it is 3kph slower than the GPS readings on my dashcams.
Cant rely on speedo which at 100kph is 6kph out with GPS. Have tried several times to correct it using VCDS but it seems Aus vehicles wont do it.

Ok it’s likely the RNS is reading/displaying the “true” speed from the ECU.

Yes speedo is way out. And all of the vehicle speeds (ECU/speedometer)will be affected by what specific tyre size you’re running.

I’ve run mine against a Vbox equivalent to be sure. I’m still not convinced that GPS speedos don’t still have an inbuilt “err on the side of caution” error so the manufacturers of these units don’t get sued.


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[Guest001]

I have the original tyres on the car and yes all speedos by law read fast and may do so by up to 10%<> 4 and may not read slow

[JustCruisn]

When they both show less than the posted speed limit you are all good.

[Guest001]

When they both show less than the posted speed limit you are all good.

Very helpful I was on cruise set at 100kph so it SHOULD have been all good Maybe it will be maybe not Will find out in 3 weeks I guess

[ian]

I always drive on the GPS speed ,i use a small tomtom in my line of sight so i don't go over the speed limit .

[Guest001]

I always drive on the GPS speed ,i use a small tomtom in my line of sight so i don't go over the speed limit .

Yes well when you have paid VW an excessive amount for a built in GPS I shouldnt need another one

[Flipper Dog]

Yes well when you have paid VW an excessive amount for a built in GPS I shouldnt need another one

True but when VW come up to the plate and offer proper camera POIs, traffic and accurate speed on their inbuilt GPS those $100 - $200 ones hold their worth.
However I must admit the traffic enabled MIB 2 on my Golf have more accurate traffic data than Navman.