Speedo Inaccurate

[Corey_R]

How much driver training does it take to read and interpret a speedo?

Apparently lots. How many people get booked for speeding in NSW each year? All the speed cameras are SIGNED and yet people STILL get booked for speeding. Obviously there are hundreds of thousands of drivers on the NSW roads who can't read and interpret their speedo correctly

[Topend]

Apparently lots. How many people get booked for speeding in NSW each year? All the speed cameras are SIGNED and yet people STILL get booked for speeding. Obviously there are hundreds of thousands of drivers on the NSW roads who can't read and interpret their speedo correctly

Do you really believe drivers speed because they can't read and interpret their speedo?

[Corey_R]

Not at all...
I think that there are many drivers who obviously speed on purpose, and I think that there are a lot of drivers that get booked by speed cameras for not paying enough attention - these drivers are helped by the fact that the speedo's now read under - but that wasn't the point I was making.

The point I was making was that pilots are by-and-large a whole lot more trained and more responsible than drivers on our roads. Comparing them is about as useful as comparing a Doctor to a check-out-chick.

[Beaker]

Beaker over reacting . . . . lol . . . never!

It won't stop me buying one now that I realise anything I buy is likely to be just as bad!


Doesn't make it right though!

[Idle]

Not at all...
I think that there are many drivers who obviously speed on purpose, and I think that there are a lot of drivers that get booked by speed cameras for not paying enough attention - these drivers are helped by the fact that the speedo's now read under - but that wasn't the point I was making.

The point I was making was that pilots are by-and-large a whole lot more trained and more responsible than drivers on our roads. Comparing them is about as useful as comparing a Doctor to a check-out-chick.

Yet, like the rest of us, pilots, medico's and checkout chicks still make mistakes — of course, doctors can often bury theirs, pilots are lucky if they make more than just one (hence their very specialised training), and checkout chicks who make too many get fired — and I've no doubt a percentage of all those also get speeding tickets, but of course then they're only drivers like the rest of us.

I can also assure you that in the course of my now rather long lifetime I've known both doctors and pilots who were not among the best of drivers.

I also know quite a few drivers who can't fly, but still don't get tickets.

None of which is a valid reason for driver aids that ought to be accurate being engineered to tell porkies.

[Corey_R]

None of which is a valid reason for driver aids that ought to be accurate being engineered to tell porkies.

Actually Idle, I think they are ALL reasons why the politicians and law makers have put these policies and rules in place around the world!

[logger]

..it's not 10% out.... it's 7% at most. If you upgrade to a car with 18 or 19" wheels, then it will come up to around 5%

...THAT is why VW have to put their cars to 7% under, as they are available with 18" and 19" wheel upgrades, it wouldn't be so easy to take your VW with a DSG down to the local mechanic and get them to change a 'gear' in the DSG to correct the error from going from 17" to 19" wheels and still having the '4km error' at 100kmph that you mention!

I do not believe "VW put their cars to 7% under, as they are available with 18" and 19" wheel upgrades".

Some misconceptions creeping in I reckon. Remember all the speed/distance measuring equipment cares about is the circumferance of the tyre. It has no interest in wheel sizes. You cannot assume that changing wheel sizes changes tyre circumference.

VW publish a list of 36 tyres that can be fitted to my 118. In reality this breaks down to 6 actual tyre sizes ranging from 15" to 18" rims. Changing from a 15" wheel and tyre combo to an approved 18" setup will change the outside circumference of the tyre by 0.003% (read zero). This will have miniscule/minute/near zero effect on your Speedos or odometer. Unless your concerned about 3km in 100,000

If you stick to VW's recommended tyre and wheel size combos you should not see ANY appreciable change to the odometer or the intentional speedo error when you change configurations. So I do not think the intentional speedo overread is there to cater for upsizing rims. I reckon it is there because for whatever reason VAG have decided, agreed or been mandated to implement it.

The distance multipliers values are listed as being vehicle specific and not wheel/tyre combo specific. ie 7 if for a Golf and 2 for a Polo and another for a Skoda, Seat and so on. They are not segregated into tyre/wheel size combos in any way.

My take on it is that VW arranged things so that the odometer will be accurately correct and the speedos will accurately over read by the intentional error whenever recommeneded wheel/tyre combos are fitted. Assuming of course you do not screw with the distance multiplier which "exists primarily to accommodate different transmission types with different gear ratios between the the driveshafts and vehicle speed sensor." I say accurately with tongue in cheek as it is German after all & it will do whatever it does accurately.

... as Coreying said they most likely won't.

Volkswagen Speedometer Error Correction

Hey Maverick are you going to correct the 3 mistakes/errors on your idea of how the speedo works page or are you still not convinced?

If you do so, while correcting 1)"Adjusting the distance impulse number does not alter the odometer, the distance impulse number is an error that is introduced for the speedometer needle." You might want to also correct 2)the assertion that the blank ---speed warning screen will show Can Bus speed. Might have been the case with a MK5 but with MK6 all this does is sets/displays current digital speedo speed as WNG speed and is nothing to do with Can Bus, which when viewed from VagCom is a totally different figure. Finally 3) I would not be so quick to write of the blokes impression that screwing with the dm can alter your indicated fuel consumption. I'd reckon if you set your ODO to overread by 8%, like I clearly showed can be done then the calclulated l/100 will get skewed in proportion. No reason to expect it would not. Appreciate it will in no way effect true consumption.

What do you reckon my chances of seeing some changes are fellas. Buckleys I reckon

[Corey_R]

You cannot assume that changing wheel sizes changes tyre circumference.

I didn't assume... I went to the tyre size calculators etc and found that changing from 225/45 R17 (VW's GTI standard) to 235/35 R19 (VW's R option) gives 1.98% change in rolling circumference. There are many people who change to to the 235/35 R19 on their GTI's etc.

Oh, and I don't think that is the ONLY reason VW underread by 7% - it's just one of the many.

Edit: Further to that, the actual change using VW options on the specific models:
GTI: 225/45 R17 to 225/40 R18 = 0.46%
R: 225/40 R18 to 235/35 R19 = 1.53%

So again - you're pretty much never going to exceed a 2% effect on the speedo by changing your wheels and sticking to the VW recommendations, and 2% is nowhere near 7% obviously, but changing wheels certainly does have an effect on the reported speed.

If you want to get even MORE specific, you can also often find the actual rolling circumference of the tyre from each tyre manufacturer, because even the same 'size' tyres can have different rolling circumferences between brands and even models of the same brand - this can also add/remove a few points to the figures above.

[Maverick]

Hey Maverick are you going to correct the 3 mistakes/errors on your idea of how the speedo works page or are you still not convinced?

If you do so, while correcting 1)"Adjusting the distance impulse number does not alter the odometer, the distance impulse number is an error that is introduced for the speedometer needle." You might want to also correct 2)the assertion that the blank ---speed warning screen will show Can Bus speed. Might have been the case with a MK5 but with MK6 all this does is sets/displays current digital speedo speed as WNG speed and is nothing to do with Can Bus, which when viewed from VagCom is a totally different figure.

The Mark 6 has a different cluster to the Mark 5 and the blank speed warning speed displays a different speed to the Mark 6 which can display the speed (unlike the Mark 5 which can't).

The testing that I performed including repeated trips over the same distances that showed no change to the odometer. I will be doing some further testing but it appears that operation has changed for the Mark 6.

Finally 3) I would not be so quick to write of the blokes impression that screwing with the dm can alter your indicated fuel consumption. I'd reckon if you set your ODO to overread by 8%, like I clearly showed can be done then the calclulated l/100 will get skewed in proportion. No reason to expect it would not. Appreciate it will in no way effect true consumption.

You mean the post where he claims that the change affects the engine operation, stops the cruise control from working in all gears except sixth and claims that fuel consumption sky rockets (no mention of the MFD reading) just that fuel consumption sky rockets. I clearly stated that this doesn't change fuel consumption.

What do you reckon my chances of seeing some changes are fellas. Buckleys I reckon

Clearly the aim of your post was not to correct any possibly incorrect information.

[logger]

I didn't assume... I went to the tyre size calculators etc and found that changing from 225/45 R17 (VW's GTI standard) to 235/35 R19 (VW's R option) gives 1.98% change in rolling circumference. There are many people who change to to the 235/35 R19 on their GTI's etc.

Yes fair enough - I will happily concede that point. Changing wheels can have, but does not have to have an effect on the reported speed. Like you say the worst case 2% is trivial compared to the over read of 6%. I was wrong when I said "If you stick to VW's recommended tyre and wheel size combos you should not see ANY appreciable change to the odometer or the intentional speedo error when you change configurations." This only applies to the TSI118 recommended tyre list.

..If you want to get even MORE specific, you can also often find the actual rolling circumference of the tyre from each tyre manufacturer.

Quite right -that is in fact what I did to establish the 0.003% difference between 225/40R18 & 195/65R15 profiles. Diameter 635 vs 637. So if you stick to TSI118 recommended tyres your wont see the anything like the differences you calculated for the GTI or R as you have shown.

Clearly the aim of your post was not to correct any possibly incorrect information.

Thats a tongue twister. Had to read it twice.... Funny thing is that it was
Oh well "I will be doing some further testing.." and "possibly incorrect information" is an excellent start. So Thanks.