Speedo inaccuracies... getting quite annoyed now...

[_AJ_]

I've still not been able to resolve this speedo issue. At 30'ish speeds I'm seeing about 5-6mph lower on the dial than it should be, and at motorway speeds it can be as much as 11-12mph over what's indicated. It's been to my nearest dealer a couple of times now, but they don't seem to know how to fix it. I called the dealer that the car came from, and was told that the TomTom I was using to compare readings with was probably at fault, which was not very helpfull at all. But, just in case, I have since checked the readings against a hand held Garmin unit, and the sat nav that's now in the car, and all the GPS based readings are whithin about 0.1mph of each other. The speedo is definately under-reading, and I'm fed up of having to second guess the speedo to make sure I don't go through 'safety' cameras too fast.

The last time my local Monaro chappy at the dealer plugged in the diagnostic machine, i was looking over his shoulder, and he showed me a facility to adjust something to do with the speed pulse. It gave a range of thousands (can't remember exact figs), and a figure that could be adjusted whithin this range. He made a small change to this number, but I've not noticed any difference, and I'm not entirely sure he know what effect it would have either way.

Can anyone help me, pretty please?

[Smiler.]

The analogue dial on mine is always out when compared to the speed indicated on an Origin B2 (GPS). Could read between 5 & 10% under on speeds up to 40mph but over at higher speeds. Even when I flashed the correct settings into the PCM, the speedo was out.

What model have you got?

[_AJ_]

It's a 6ltr VXR 500... all the cars I've had in the past have over read slightly, and you'd expect things to be manufactured on the safe side, but I'm fairly sure there's a rule written somewhere that says it can't under read.

[Mattmeister]

mine 'under reads' too! Was recently scamera'd and im sure the speedo wasnt showing what the NIP showed! for 3 points it hardly seems worth trying to prove it tho

first noticed because at VMAX I got a lazer trap indicated 165 whereas the speedo was showing only around 155!

[greens vauxhall]

Mattmeister said:

mine 'under reads' too! Was recently scamera'd and im sure the speedo wasnt showing what the NIP showed! for 3 points it hardly seems worth trying to prove it tho

first noticed because at VMAX I got a lazer trap indicated 165 whereas the speedo was showing only around 155!

The speedo will never show more than 155/6 ish, car goes faster, speedo doesnt!!

[V8 EOL - Rich]

greens vauxhall said:

The speedo will never show more than 155/6 ish, car goes faster, speedo doesnt!!

Errm, I have had 167 out of mine... and yes MPH, not KPH.

[ringram]

You can tune it out in the PCM. Earlier ones (Like my VT2) had a little electronic bit to do the work. But the PCM has all the gear built in, as expected as it deals with various factory gear ratios. I know Stigmund had a problem which he fixed with a different speedo cal in his tune.
Simple change for anyone with a tune tool.

[Doleboy]

The PCM has a Vehicle Speed Sensor (VSS) and measures the number of pulses the speed sensor produces per mile.

This value is used to determine the vehicle's road speed and speedometer needle in the instrument cluster. Increase this value to retard the speedometer needle, decrease this value to advance the speedometer needle.

If you'd like to double check the changes your local dealer made (I'm guessing using the GM TechII tool) then you could download the demo version of EFI Live and use the "Speedo Calculator" tool calculate the correction factor you'll need to apply against the current value.

So, if your speedo reads 30 pmh when you're really doing 33mph the correction factor should be 0.909091 or if your speedo read 70mph when you're really doing 82mph then the correction factor should be 0.853659

Adjust the current number Pulses per mile setting by the correction factor and away you go (in theory)

Hope that helps.

[ringram]

Exactly correct Sir

Almost worth buying efilive for...

Edited by ringram on Monday 24th September 22:00

[Doleboy]

ringram said:

Exactly correct Sir

Almost worth buying efilive for...

Edited by ringram on Monday 24th September 22:00

I wonder....

Edited by Doleboy on Monday 24th September 22:11

[_AJ_]

Doleboy said:

The PCM has a Vehicle Speed Sensor (VSS) and measures the number of pulses the speed sensor produces per mile.

This value is used to determine the vehicle's road speed and speedometer needle in the instrument cluster. Increase this value to retard the speedometer needle, decrease this value to advance the speedometer needle.

If you'd like to double check the changes your local dealer made (I'm guessing using the GM TechII tool) then you could download the demo version of EFI Live and use the "Speedo Calculator" tool calculate the correction factor you'll need to apply against the current value.

So, if your speedo reads 30 pmh when you're really doing 33mph the correction factor should be 0.909091 or if your speedo read 70mph when you're really doing 82mph then the correction factor should be 0.853659

Adjust the current number Pulses per mile setting by the correction factor and away you go (in theory)

Hope that helps.

That sounds like exactly what I'm looking for, thankyou. Once I've read it a couple more times and got it clear in my head, I shall pass the info on.

[stigmundfreud]

Doleboy said:

ringram said:

Exactly correct Sir

Almost worth buying efilive for...

Edited by ringram on Monday 24th September 22:00

I wonder....

Edited by Doleboy on Monday 24th September 22:11

do get it but then speend many waking hours wanting to get things sorted, it takes time to get there and then yo get it srted, change something on the car and back you go

[iacs_hsv]

ringram said:

...Earlier ones (Like my VT2) had a little electronic bit to do the work.

Rich, I'm considering removing the divider from my VX and re-calibrating via the PCM - have you already done this mod to your clubby?

[ads_green]

Calibrating the speedo is best done with new fresh tyres inflated to the correct pressure.

As the tyre wears or loses some pressue the speedo will drift - at least if you calibrate the speedo with tyes set correctly then it will drift into slight over-reading.

[ringram]

Hi iacs_hsv, yes I did this as the piece on mine died making my speedo die. I simply removed it and put the purple wire back as it was factory, then changed one of the pcm values. It now reads correct on the dial as well as the vss signal from pcm giving correct speed in kmph or mph. Some info on here including pics http://r8transformation.blogspot.com/
Stig is right its out a bit factory. I think most cars are. But you can make it exact if you like same values get changed either way.

[stigmundfreud]

ringram said:

Hi iacs_hsv, yes I did this as the piece on mine died making my speedo die. I simply removed it and put the purple wire back as it was factory, then changed one of the pcm values. It now reads correct on the dial as well as the vss signal from pcm giving correct speed in kmph or mph. Some info on here including pics http://r8transformation.blogspot.com/
Stig is right its out a bit factory. I think most cars are. But you can make it exact if you like same values get changed either way.

some of the settings in the tables have no effect on m6 cars though. With regards to this divider is that something they bolt onto the uk monaro perhaps? The thing that tends to die and send the dials erratic or is that another area?

[odyssey2200]

Have you changed the wheels from standard??

[ringram]

Nope standard wheels, but 245 tyres instead of 235's for me.

[odyssey2200]

ringram said:

Nope standard wheels, but 245 tyres instead of 235's for me.

so the tyre is wider than standard but if you kept the same profile the rolling radius is actually slightly larger

eg 50% of 245mm as opposed to 50% of 235mm

Tyre is therefore 100 taller py times r etc

So at any given speed your wheels are actually going arouns slightly slower than standard hence the speedo reads slow.

[ringram]

Yeah I know dude. Mine is fine, Im not the one complaining, just explaining