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Wheel/Tyre Size and Speedometer adjustment.


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Off the back of a recent thread where we got onto the topic of wheel and tyre size, and the changing of same to make things more comfortable I decided to do some digging into how the speedometer might be corrected if larger tyres are fitted.

 

Fundamentally, this involves changing what VAG refer to as the K-number. This is a parameter that tells the cluster how to translate the signal from the wheel speed sensors into the correct km value for display on the speedometer and odometer. On older cars this was directly editable in an adaptation channel. Cursory glances at the Superb's cluster previously suggested this wasn't possible here.

 

Given the Superb's lineage, I figured starting with a search for information on the 1K Golf and 3C Passat might yield something. The following links were useful as a guide on what to go looking for:

http://wiki.ross-tech.com/wiki/index.php/VW_Golf_(1K)_Instrument_Cluster

http://forums.tdiclub.com/showthread.php?p=3393030#post3393030

 

This suggested that the K-number was set via a coding change, and this correlated with what I'd found before,but now I had a better idea of what to look for.

 

So tonight I went out to the car with VCDS in hand and did some digging.

 

The first thing to note is that the Superb's cluster has 9 coding options for the K-number, some of which are labelled for certain platforms. The default coding was 1 which had a list of Skoda and SEAT models associated with it, the Superb 3T included. Going through Advanced Measuring Blocks, I found the blocks for K-Number and Wheel Circumference: 22084 and 1947 mm.

 

The 225/40R18 tyre size has a nominal circumference of 1922 mm; 225/45/R17 is 1913 mm. Close enough :)

 

I figured it was likely that I'd have to change coding to figure out what the rest of the values are so away I went. Changes to 0, 3, 4, 5, 6, 7, 8 were rejected. None of these coding values had Skoda models listed in the description: I suspect there's some sort of lock in place to prevent values for non-Skoda models being selected.

 

Value 2 had Skoda models listed in it so I switched to that. The corresponding K-Number and wheel circumference were 21224 and 2025 mm. The 235/45/R18 tyre size has a nominal circumference of 2017 mm. Again pretty close.

 

I'd wonder if there's an adaptation value to allow minor correction from the coded wheel circumference to compensate for the slightly smaller size of the actual tyre.

 

So if you're switching to 235/45/R18s on your Superb changing the K-number coding to 2 will correct for the change in rolling radius.

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All the coding counts for nothing if you just go by the size / numbers on the sidewall like 225/50 R17

The reason being you will need to measure one Brand & Tyre type accurately and get the rolling circumference because the same sized tyres new from different manufacturers can measure differently.

Then there is tyres as new compared to worn or how the pressures are on the tyres.

 

I use 205/40 R 17 tyres that physically are bigger than 215/40 R 17 tyres as the 215 is the width of the tyre not the tread on the road, and the 40% aspect does not actually appear to be correct in reality.

http://kouki.co.uk/utilities/visual-tyre-size-calculator

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All the coding counts for nothing if you just go by the size / numbers on the sidewall like 225/50 R17

The reason being you will need to measure one Brand & Tyre type accurately and get the rolling circumference because the same sized tyres new from different manufacturers can measure differently.

Then there is tyres as new compared to worn or how the pressures are on the tyres.

 

I use 205/40 R 17 tyres that physically are bigger than 215/40 R 17 tyres as the 215 is the width of the tyre not the tread on the road, and the 40% aspect does not actually appear to be correct in reality.

http://kouki.co.uk/utilities/visual-tyre-size-calculator

Sure those variables exist, hence why I quoted nominal values for wheel circumferences. But those are going to vary around that nominal value for any given tyre size.

The biggest variation in rolling radius/circumference is going to come from tread wear over the life of the tyre: this will be a higher percentage of the original value as the outer radius gets smaller.

 

FWIW the calculator I was using for the nominal circumferences in my OP (at http://www.carbibles.com/tyre_bible_pg4.html) calculates the effective rolling circumference (based on the rolling radius) rather than the unloaded circumference of the tyre.

 

Regardless of any opinions any of us hold on the topic of wheels and tyres (and there are lots of them) there are people who want to keep their 18s and choose the larger tyre size. This information allows them keep Skoda's intended calibration on the speedometer/odometer.

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chimera,

i change quite a lot of tyres on quite a lot of cars and also check with GPS and V-Box and i know how not accurate Skodas Intended calibration 

is of speedometer / odometer.

They are often differences between the Odometer that shows and the one for trip by a few tenths.

So it is not my opinion just experience and practice.

 

It is good to know that you can get the reading more accurate than 'those Skoda intended to get, but failed to quite often, but then obviously 

get closer with sometimes. New tyres, recommended pressures, or tyres that measure much as the scores on the doors indicate they do.

 

PS

A tape measure run around the tyre is much more accurate than any table that gives a 'calculation'.

If you have different tyres / brands / tread types but with the same size and profile on the side wall maybe run a tape around them, and as new as a comparison.

 

It is much as you do when setting a computer on a push bike, 

measure how far the tyre actually rolls for the size.

'Close enough is not really good enough often enough'.

Edited by Offski
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