Will my 20” wheels affect my speedometer?

[The Flying Scotsman]

I’ve got 20” wheels with 275/40/20 tyres.
I’m pretty sure my speedometer hasn’t been calibrated since.
Will my speedo be out?
I’ve noticed sometimes I seem to be faster than the traffic sitting at the speed limit.
What diagnostic tool is used to calibrate the speed VCDS or Carista?
Cheers

 

[Bav]

If you know the wheel/tyre sizes it was running with previously, you can use this site to determine the impact on your speedo...

Alloy Wheel & Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Alloy Wheel & tyre fitment calculator - it calculates inset, poke, speedo error, tyre stretch and loads more.

www.willtheyfit.com

You never know, your speedo might actually be closer to reality now than it was.

 

[cgtmiles]

The answer is almost certainly yes, if you compare the size against the standard you can get the mph difference from the above tool. My 19s give a few miles over the standard but actually that works out as almost exactly accurate as the Speedo reads over anyway. I used a gps speedo app to give me an idea of the actual speed.
You could try the green/red speed traps common when entering a 30, stick to 30mph and see if you get a happy green smilie and an angry red one

 

[EAN]

Do you have a T6 or a T6.1?
My T6.1 on standard 205/65R16‘s reads 68mph at a true 70mph (digital centre read out vs GPS), so pretty accurate out of the box.
I understand that once the values on a T6.1 are changed they cannot be undone.
It totally put me off going with a larger rolling diameter.
There is a thread on here somewhere.

 

[The Flying Scotsman]

Do you have a T6 or a T6.1?
My T6.1 on standard 205/65R16‘s reads 68mph at a true 70mph (digital centre read out vs GPS), so pretty accurate out of the box.
I understand that once the values on a T6.1 are changed they cannot be undone.
It totally put me off going with a larger rolling diameter.
There is a thread on here somewhere.

T6 startline

 

[The Flying Scotsman]

If you know the wheel/tyre sizes it was running with previously, you can use this site to determine the impact on your speedo...

Alloy Wheel & Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Alloy Wheel & tyre fitment calculator - it calculates inset, poke, speedo error, tyre stretch and loads more.

www.willtheyfit.com

You never know, your speedo might actually be closer to reality now than it was.

I had the 20’s fitted before I picked up the van. So not entirely sure of original tyre sizes.

 

[Bav]

...It totally put me off going with a larger rolling diameter.

An increase in wheel diameter doesn't necessarily result in an increase in rolling diameter though - it depends on what tyre sizes you are/were running with.

 

[Bav]

I had the 20’s fitted before I picked up the van. So not entirely sure of original tyre sizes.

Do you know the original wheel size? Some T32s came with 17" wheels; otherwise they come with 16" - someone on the forum should be able to confirm the standard, factory tyre sizes used for either of these wheel sizes.

 

[T6 Syd]

Instructions for adjusting here, credit to @mmi , check the current situation against a gps if you can to see whether it does need adjusting. On my T6 it took a bit of guesswork, but the coding is reversible on that model so you can try multiple times.

Module 17 – Instruments carries also the coding to select a few scaling factors for speedometer.

Byte 0, bits 0..3 have mysterious value “K-number”. It turned out changing this coding adjusts measuring values IDE00842 (K-number) and IDE00845 (Wheel circ. mm) of module 17 - Instruments, and thus adjusts speedo scaling.

View attachment 20070
I run through all the above coding combinations 01-07 and read the measuring values for each coding combination. The results are in the table below.

View attachment 20072

Speedo can be adjusted simply by changing the coding. Resulting speedometer delta (vs. factory setting) can be seen in rightmost column, which is actually just a calculation of “corrected” wheel circumference vs. factory value (grayed line).

However, there are some indications that also other factory settings exist...

​

In the case above the change can only make speedo read higher as the van came from the factory with tiny 16” wheels and coding is already at the "lowest" setting of “6”.​

​

Besides factory setting I verified coding values 3 and 4, and indeed speedo is reading way more over. For example, with coding “3” driving at 100 km/h (GPS-speed) the speedo was about 112-113 km/h. At factory coding "6" at 100 km/h (GPS-speed) the speedo reads about 102-103 km/h. As expected these match with the table above (about 9% difference). I think this already is a quite good confirmation that delta values in the table work for the purpose.​

Hopefully members can confirm the table above or just parts of that. Especially interesting would be to see how vans with factory wheels of 17” or 18” are coded and if the measurement values IDE00842/IDE00845 vs. coding are the same or different. Also if any unexpected side effects can be seen in changing the coding… would definitely be nice to know.

​

Just for the record that in VCDS adaptations is a channel “ENG112416-IDE01766 Velocity detection – Distance impulses”, which has value “4”. Changing that didn’t have any effect on above mentioned measuring values. So the purpose of this channel remains unclear, hence request to consider any side effects.​

This tweak is an adaptation from Speedometer Calibration - Page 3

------------------------
Edit:
Added a table from a different van.
View attachment 20659

---

EDIT (2022-04-15)

*** NOTE!!! This modification is NOT applicable to T6.1 ***
*** IRREVERSIBLE on T6.1 ***​

T6.1 electronics are based on MQB architecture thus very different than T6.


Unfortunately there are indications that the speedo adjustment on T6.1 is a one-time deal - thus irreversible View attachment 153106
Please have a read of the following:

VW Transporter T6.1 2021 Wheel size coding changes issue

Hiya folks! A customer of mine changed his wheels to 20" and wanted his speedo coded to suit the new wheel size. No issue as this is an available change via the "K-Number" coding in 17 Instrument Cluster and done this many times in older Transporters and Amaroks. Well, turns out there is an...

forums.ross-tech.com

2020 MAN TGE (VW CRAFTER2), Coding BYTE3 in Control Unit J285 can not be undone

Hello VCDS community, first I have to thank Uwe for this great tool (VCDS), which gives me so many opportunities with the car. I love it. Thank you very much for that. :thumbs: A short introduction to my person. My name is Thorsten and I am an electronic engineer and computer guy from Germany...

forums.ross-tech.com

 

[EAN]

An increase in wheel diameter doesn't necessarily result in an increase in rolling diameter though - it depends on what tyre sizes you are/were running with.

@Bav correct. I’ve just ordered some 18” and will be fitting 255/45’s to keep my rolling diameter the same.
Easier to increase the overall size with a T6 and keep the speedo accurate than with a T6.1

Edit: Unfortunately the wheels I’ve just purchased are 7.5J so the max tyre width is 235
So, in my case, I will need a 235/50/18 to keep the same rolling diameter.

 

[Bav]

Easier to increase the overall size with a T6 and keep the speedo accurate than with a T6.1

How so?

 

[EAN]

How so?

@Bav My understanding is for wheel size you can code and recode a T6 but on a T6.1 you can only code one way and can’t undo it.

 

[The Flying Scotsman]

I suspect the originals were 16s.
I’ve downloaded a speedometer app so will check and see how accurate my speedo is.

 

[Bav]

I suspect the originals were 16s.
I’ve downloaded a speedometer app so will check and see how accurate my speedo is.

The factory tyres were probably 205/65R16s then, which gives a relative speedo underrating of 7.57% (i.e. when speedo shows 60mph, you're actually doing 64.5mph). Although, speedo was probably over-reporting originally, so real difference would be less.

 

[temporarysecretary]

I suspect the originals were 16s.
I’ve downloaded a speedometer app so will check and see how accurate my speedo is.

If you get an original tyre/wheel size from the driver's door. Then compare it to you size with this.

Alloy Wheel & Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

You are probably +8% different to stock.

So your speed will read 8% lower than stock calibration.

MPG -8% lower.

and mileage count 8% slower.

For me on That size 70mph real world reads as 70 on the clock. You need to remember that so if you wander over that spoed you have no safety net.

 

[Bav]

...and mileage count 8% slower.

Not all bad then.

 

[temporarysecretary]

Not all bad then.

Also means to bear that in mind for service intervals and when buying a van. If fitted for the life of the vehicle it does add up.

 

[cgtmiles]

I suspect the originals were 16s.
I’ve downloaded a speedometer app so will check and see how accurate my speedo is.

You can use the tyre pressure sticker on the driver door to give an idea of the original wheels but you will only know the difference between that and the standard, a gps speedo should give you an idea of what speed you are actually doing because as mentioned the standard can give quite some variation in readings anyway

 

[Bri1701]

I was always lead to believe that from the factory any car or van speedo was not allowed to read under the actual speed. But anything up to 10% over is acceptable.

i.e. Actual speed of, say, 70 mph - speedo reading anything up-to 77 is ok

Same actual speed of 70 mph - speedo reading, 69 or lower is not ok

Edited to hopefully make sense!

 

[The Bear78]

Just check it against GPS to see if there is any need to adjust it.
You can run Sat nav from Google Maps and it will give live speed information.
No standard speedo should under read ie. you're actually going faster than it says. Nearly all of them will be optimistic which is a useful safety net.



This is the change I made from standard sizes to 235/65/17. The speedo is now bang on. FYI the calculator assumes that your original sizes are matched to the speedo...