Speedo Inaccurate

[Topend]

Coreying,

Thanks for all your input. You have provided me advice on many of my questions which is appreciated.

Cheers,
Dave.

[logger]

Sort of. But take the aircon speed out of the equation. It comes from the CANBUS speed which is derived from the ABS individual wheel speeds. This should always be very accurate and only varies with tyre diameter (and slip). It is independent of the speedo/odo system and should remain unchanged through out.
The three speeds you change by buggering with the Distance mutiplier are the digital speedo the anologue speedo and the odometer. Change the distance multiplier and these three speeds will ALL change accordingly. With correct distance multiplier set ODO will be accurate and 2 speedos will overread by design.

The only instrument affected is the speedometer. The multiplier increases the value going to the to the analog speedometer and no other instruments. It doesn't touch the odometer at all.

Thats rubbish Mav. It definitely effects all three items. The digital speedo the anologue speedo and the odometer. As I said, Change the distance multiplier and these three speeds will ALL change accordingly.

[brad]

As I said, Change the distance multiplier and these three speeds will ALL change accordingly.

Not hard to test if you have VCDS.

Who's up for it?

[logger]

Not hard to test if you have VCDS.

Who's up for it?

As you rightly say, it is not hard at all to test. Have already done so. My logs from this testing confirms the view of the makers of VCDS. This is how I know Mavericks assertion are wrong. Will post some stuff later to show how changing the distance mulitiplier skews the odometer and both speedos..

[brodzta_GTI]

as previously stated.. its acceptable far as VW is concerned.. we have a copy of the mathamatical equation at work thats used by all manufactures to me ADR to find "true speed".. it also states that you CANT use a gps to find out how fast your going..
Ive got a 01 gti and gps says it 7kms under.. if it helps you avoid a speeding ticket... thumbs up in my books

[cme2c]

as previously stated.. its acceptable far as VW is concerned.. we have a copy of the mathamatical equation at work thats used by all manufactures to me ADR to find "true speed".. it also states that you CANT use a gps to find out how fast your going..
Ive got a 01 gti and gps says it 7kms under.. if it helps you avoid a speeding ticket... thumbs up in my books

Hi, brozda_GTI. Where's work? I understand that a GPS can't be used as evidence in court, but MOST of the time they are very accurate. The inaccuracies happen when they change satellites. I went under a bridge on the M5. The GPS momentarily reported me doing 400 Km/hr! Since I wasn't in a Veyron I thought this was unlikely.

[Maverick]

Hi, brozda_GTI. Where's work? I understand that a GPS can't be used as evidence in court, but MOST of the time they are very accurate.

GPS can be used as evidence in court if you record the HDOP and depending on what the car is doing at the time has a decent update interval (5Hz is a good interval). If you have a video with this device providing the overlay you won't have any problems with it being accepted in court as long as the HDOP is <1.00 (or not far from it, the video helps as it shows what the movement is like and can be used with the GPS time to verify the GPS accuracy if required by using points of reference on the road.

GPS tracking systems with no HDOP and with logs that are updated only a few times a minute have been used in court before on both sides and been accepted so by using something like the system above it's very unlikely you would have any problems.

The inaccuracies happen when they change satellites. I went under a bridge on the M5. The GPS momentarily reported me doing 400 Km/hr! Since I wasn't in a Veyron I thought this was unlikely.

The inaccuracy has little/nothing to do with changing satellites, the GPS connects to as many satellites as it can but it has everything to do with the location of the satellites. The DOP will be high (which is poor) if they're too close together (this can occur in cities for example) whereas the best DOP is if the satellites are far apart which gives a low DOP.

If you have a high DOP value you won't have a 3D fix so the GPS won't know if you're doing up or down a hill which affects the accuracy even further.

[logger]

OK this is how I tested to confirm changes to the instrument cluster distance multiplier value effects all three speeds. Odometer, digital and analog speedos.

I drove my MK6 TSI with all 8 possible distance multiplier values set and measured various parameters. As it is not practical to find an accurately measured known distance, of worthwhile length, I did the reverse. I drove in CRZ control at constant GPS speed of 100kph for a fixed time of 6 minutes. In other words I drove 10km at 100km as accurately as possible and compared Trip distance on the ODO to the known 10km distance. (Trip meter measures to tenths whereas ODO itself whole KMs only.)

Before any naysayers say, the trip meter is not the ODO, I covered the possibility that one may diverge from the other by starting Trip meter exactly as ODO rolled over, noted reading and did not reset until end of testing 234km later at which point ODO/Trip still matched exactly. They do not diverge from one another when you change the multiplier. They are both effected the same.

So I would set a distance multiplier value with VagCom, accelerate to 100kph on GPS and engage CRZ control, let it settle down and then as a whole KM passed on the Trip meter start a stop watch and time for 6 minutes or in a couple of cases only 3 minutes. As I was driving on flat country roads during each test the speed would occasionally creep up to 101 or drop to 99 on the GPS. But it averaged out well over the time period.

The results where decisive. First I tested the car with the stock value of 7 and after 3 minutes at 100kph GPS with both speedos showing 106 the trip meter incremented exactly 5.0km. So my Odometer is accurate! Did this twice and both times results were identical
Then as I had some clearer stretches of road I was able to do six consecutive 6 minute x 10km test runs. First I tried the dm value 1 as it was the other end of the scale from the shipped value of 7. Two consecutive tests showed both speedos read 114kmh at true 100kph GPS and trip meter increment 10.8 km. So toggling the dm from one end of the scale to the other induced an 8% over read into the ODO as well as increasing the speedo error markedly.

Went on to test distance multiplier values 4 & 6 over 6 minutes as well . As expected the the further the value diverges from the shipped value of 7 the greater the resulting ODO and speedo error. The induced ODO error for 6 was on 2% whereas for 4 it was 3%.

So this categorically shows how stuffing with the distance Multiplier via Vag Com will also screw with your Odometer.

As it happens my car a MK6 TSI, is factory set at 7 on a scale from 1 to 8. If you go set any value lower than 7 all you achieve is to make you speedos over read more than they already do and cause the previously accurate Odometer to overread by anything up to 8%.

In the underread direction there is only one value available which is 8. Setting this will make both of my speedos read 104 at 100kmh slightly reducing the intentional known error. Although I have yet to test it, it will no doubt invoke a slight ODO underead error as well. Edit... Tested now and can confirm dm8 indded gives an ODO underead of 1% as expected

So on a MK6 TSI it is a waste of time screwing with distance multipliers in Vag Com in an attempt to fix your speedo error because it cannot be done.

Lucky for me I remain happy to have my Speedos indicate 106 at 100 true.

Here is a table showing how changing the distance multiplier effects things on a MK6 cluster.

Dist.		Speedo		Speedo	true  ODO	
mult.	gps	ovread	error	@100	dist	dist	error  %
1	100	114	14%	87.7	10	10.8	0.8	8.0%
2	100	113	13%	88.5	10	10.7	0.7	7.0%
3	100	110	10%	90.9	10	10.4	0.4	4.0%
4	100	109	9%	91.7	10	10.3	0.3	3.0%
5	100	108	8%	92.6	10	10.25	0.3	2.5%
6	100	107	7%	93.5	10	10.2	0.2	2.0%
7	100	106	6%	94.3	10	10	0.0	0.0%  <- default
8	100	104	4%	96.2	10	9.9	-0.1	-1.0%

Anyway I trust this is all of interest.

[Corey_R]

Thanks logger

[Topend]

Nice work logger.