Hi!

Has anybody else noticed that the speedo in their SM is inaccurate? I have made comparisons against my TomTom (which I believe to be fairly accurate), and the SM speedo is off by a lot more than the Roomster I had previously. When I set the cruise to exactly 80 kmh on the TomTom the dial shows 86, and the digital info shows 85, in other words the speedo is wrong by over 3 mph - and this is only at 80 kmh (50 mph). At higher speeds the error is even bigger.

I have a 1.2 TSI with 225/50 tires on 17 inch rims, the standard tires on an Adventure (SE) spec SM.

And does anybody know if and how this can be corrected via VCDS as I have a Rosstech cable available?

Sounds ok to me, at an 80MPH indicated speed on my old octavia Tomtom said the speed was just over 70MPH which is a near 10mph difference. As you go faster the error increases.

The satnav is always going to be the accurate one as your speedo can be anything up to 10% faster than what you actually doing according to auto experts on the radio on thursday about this very subject.

Set it by your satnav if you reset it.

:yes:

Speedos are always set to read fast . They are made to a tolerance and to protect the motorist from unnecessary speeding convictions are set to read fast. Also factors like tyre wear will affect readings.

The satnav is always going to be the accurate one as your speedo can be anything up to 10% faster than what you actually doing according to auto experts on the radio on thursday about this very subject.

Set it by your satnav if you reset it.

:yes:

Doesn't that depend on how accurate your satnav is??

Doesn't that depend on how accurate your satnav is??

Satnavs are extremely accurate going in a fairly straight line at a steady speed.

If you change direction and speed fairly dramatically the satnav takes a short time to catch up - my Tom tom does anyway.

OOOOO thanks for the info, I'll remember that :yes:

Construction and use regulations specify that speedos are not allowed to be slow at all but can have a tolerance of up to 10% fast.

It is cheaper and easier to err on the fast side than to risk falling foul of the regulations.

Hi!

I am aware of all these factors that leads to speedos usually showing too much. I was just surprised that the error is so big since the speedo in my Roomster wasn't off by more than a mile or so - and I got used to seeing the actual speed in the dash.

My testing compared to the Satnav was done on straight flat roads using the cruise control, so the satnav speed is the accurate one.

Does anybody know if it is possible to reset/calibrate/whatever the speedo using VCDS?

Edit: I was out cruising today and did some test with the MFD, and the average speed shown in the MFD is more or less spot on! I tested with 60, 80 and 100 kmh, and the difference between Satnav and MFD was 1 kmh!. I tested with cruise control on highway stretches so the testing is valid.

Edited 4 July, 201016 yr by the_raz

Do not expect high accuracy of your speedometer ...

The precision set at design time is always the order of 5 or 10%.

I worked for over 10 years in automotive design office (try to guess which brand? ) and I have personally seen this.

Never exist, on a common use car, a speedometer with 100% accuracy.

Imagine what would happen if challenged by an offense for speeding.

Police said one speed, the driver said that his speedometer marked 1 km / h less ...

then at the end who pays the fine, the Skoda because of their speedometer??

It's more easy to install devices which mark a greater speed instead of a more accurate one.

Edited 4 July, 201016 yr by medi@wiz

Does anybody know if it is possible to reset/calibrate/whatever the speedo using VCDS?

No because it has to be fixed, as specified by pan Europe specification.

...Has anybody else noticed that the speedo in their SM is inaccurate? I have made comparisons against my TomTom (which I believe to be fairly accurate), and the SM speedo is off by a lot more than the Roomster I had previously. When I set the cruise to exactly 80 kmh on the TomTom the dial shows 86, and the digital info shows 85, in other words the speedo is wrong by over 3 mph - and this is only at 80 kmh (50 mph). At higher speeds the error is even bigger.

I have a 1.2 TSI with 225/50 tires on 17 inch rims, the standard tires on an Adventure (SE) spec SM.

And does anybody know if and how this can be corrected via VCDS as I have a Rosstech cable available?

At true 70 MPH the indicated instrument speed is 74/75 MPH the tolerance is set to +10% by the manufacture to allow for tyre wear so at true 70 MPH you could have 77 MPH shown.

The SAT NAV tolerance is 0.01% depending on what system is being used. Driving today without Tommy boy is points suicide....

JD

I noticed on the maxidot menu a change setting for winter tyres? Is this to reset the speedo for different tyre size, or does it change the traction settings?

It had a percentage seting, could this be used to calibrate the speedo correctly?

I noticed on the maxidot menu a change setting for winter tyres? Is this to reset the speedo for different tyre size, or does it change the traction settings?

It had a percentage seting, could this be used to calibrate the speedo correctly?

Nope, that setting simply enables an audible alert when you reach the max speed rating of a winter tyre and does nothing to the speedo accuracy. As already mentioned this is set at the factory and is not adjustable, as per EU specification.

Nope, that setting simply enables an audible alert when you reach the max speed rating of a winter tyre and does nothing to the speedo accuracy. As already mentioned this is set at the factory and is not adjustable, as per EU specification.

Sorry but I think you are also wrong.

The Yeti Maxidot does allow a percentage change to be made to speedometer and mileoneter readings to take account of the different rolling radius when fitting winter wheel/tyres. I have tried it and this seems to work although without actually changing the wheels/tyres - yet.

The speed limit warning is separate and can be adjusted to whatever speed you wish entirely independant of the % change function.

Edited 11 July, 201015 yr by Trevorminor

I believe in Germany the tolerance is only 3% on cars built since 2000. My TT is absolutley bob-on according to my Garmin Sat-Nav and Roadhawk cameras. 70mph on the dash is 70mph. Which is a bit of a shock after routinely driving everywhere at 75mph for the last 20 years.

Having had 4 x new tyres on my other car (dealer quote £940, local specialist - £771...... gulp !) - remember the rolling circumference of the tyre will change over time from "new" to "replace".

Having had 4 x new tyres on my other car (dealer quote £940, local specialist - £771...... gulp !) - remember the rolling circumference of the tyre will change over time from "new" to "replace".

But that should be safe, as the tyre wears the car will be going slower on the road as it covers less ground for the same wheel revolutions. If the speedo isexactly correct for the new tyre it will slowly read faster as the tyre wears, making the initial built in innacuracy even worse if not corrected.

This change is far less than the original innacuracy seen. I did a quick calculation, assuming 6mm wear from new, and the difference in circumference is in the order of 1% between a new and worn out tyre

But that should be safe, as the tyre wears the car will be going slower on the road as it covers less ground for the same wheel revolutions. If the speedo isexactly correct for the new tyre it will slowly read faster as the tyre wears, making the initial built in innacuracy even worse if not corrected.

This change is far less than the original innacuracy seen. I did a quick calculation, assuming 6mm wear from new, and the difference in circumference is in the order of 1% between a new and worn out tyre

Agreed. I had to explain the content of this thread to my boss a few years ago, as he was looking to discipline someone for claiming excess mileage on their expenses ! (e.g. 205 mile trip, but claiming 210...).

I wouldn't mind too much, but he was an engineering director !

The 225/50-17 tyres are 1 inch larger in diameter than the 215/60-16 resulting in a 1.1% higher speed reading for the 17 inch standard tyres.

Assume the thread wear to acceptable replacement condition is 6 mm, the diameter will have been reduced by ½ inch or about ½% slower speed than read.

215/65-16 inchtyres would be 3.2% higher speed at equal speedometer readings to the satndard 21/60 - 16 size or 2.2% highe speed than the 17 inch.

Probably no room for chains, but maybe not a bad selection for winter use? The speedo reading should be closer to actual than the standard tyres.

DK Fuzz have an automatic 3% deduction from their measured velocity.

AARRGGHH!!!

He mentioned chains!!

AARRGGHH!!!

He mentioned chains!!

Verrry Sorrry Llanigan, but if not, it was sure to pop up

The 225/50-17 tyres are 1 inch larger in diameter than the 215/60-16 resulting in a 1.1% higher speed reading for the 17 inch standard tyres.

Assume the thread wear to acceptable replacement condition is 6 mm, the diameter will have been reduced by ½ inch or about ½% slower speed than read.

215/65-16 inchtyres would be 3.2% higher speed at equal speedometer readings to the satndard 21/60 - 16 size or 2.2% highe speed than the 17 inch.

Probably no room for chains, but maybe not a bad selection for winter use? The speedo reading should be closer to actual than the standard tyres.

DK Fuzz have an automatic 3% deduction from their measured velocity.

Surely this is taken into account in the car build with the correct setting put in for the wheels used. I know in the good old days you were supposed to change the gear in the gearbox for the speedometer drive if you went to a different wheel size. This was one of the things you had top do when substituting secondhand gearboxes from one model of car into another. Now I gues it is a straight change to the management settings.

My previous car should have had 16" wheels, and I went for the oprtional 17" ones from new and all the sppedo reading were as expected with the normal error.

Just as an aside, if speedos are about 5% out the mileage covered will be indicated as more than actual: This means that all fuel consumption figures calculated from fuel bought v. mile covered are 5% optimistic, and we all service our cars 5% early giving extra trade tm the dealers! I have also come to the conclusion that the maxidot average fuel figures are very optimistic as well. Mine indicate 51mpg for the last tankfull, yet brim to brim suggests it was really around 46mpg! so real mpg is probably closer to 44mpg, as against the 51 shown, ie about 14% incorrect.

Most OEM wheels ( say 15, 16,17,18" ) are fitted with different profiles of tyres so that the overall rolling diameter is about the same within fairly close limits. Thus the manufacturer can keep the gearbox ratios the same for different models.