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aquandes
Member Since: 11 Feb 2020
Location: Marin Epagnier
Posts: 10
  
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| non standard tyre size impact on autobox |
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Beginning of July, my right front tyre developped a bulge resulting in vibration and steering reaction. A solution was needed ASAP.
Problem: no 255/55R19 could be found in Switzerland, not even second hand.
Decision was made to order new set and the mechanic lent me his winter tyres that were 255/50R19.
Delivery took 6 weeks, so I had time to clock a few thousand miles with the winter tyres.
Apart from the evident correction on the speedometer, the gearbox changed its behaviour.
Going uphill, it was hesitant and up shifting was off the optimum rev range.
When I got my tyres fitted, it all came back to normal.
This small story shows how a slight change in wheel diameter can affect an electronic gearbox.
I didn't have that kind of problem with my 6 cylinder 109 on 9.00/16 tyres 
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 11th Sep 2020 7:04 am |
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al cope
Member Since: 08 Nov 2005
Location: Oldbury, WM
Posts: 10349
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I ran 265/50x20's on my D3 auto and D4 auto, with no apparent change in how the autoboxes seemed to be working
Al Volvo XC90 B5 Plus Dark
Gone - MY18 D5 HSE - Corris on 22's with Black Pack
Now gone - MY16 D4 SE Tech, Loire Blue, Almond Leather, Privacy, plus some other goodies.
Old - MY12 D4 SDV6 XS Auto - Ipanema Sand with Almond Leather - Plus other niceties, and D4.com sticker
Older - D3 TDV6 XS Auto - Lugano Teal with Almond Leather, 20" Stormers, Shiny Tailpipes, DVD/TV - and obligatory D3 sticker
Ancient - D3 TDV6 S - Tonga with Ebony, 20" Stormers, satnav & DVD
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| 11th Sep 2020 7:14 am |
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aquandes
Member Since: 11 Feb 2020
Location: Marin Epagnier
Posts: 10
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Theoretical wheel diameter, compared to standard size is:
97% for the winter tyres I ran
101% for the 20" tyres you run
either it's not noticeable for larger sizes, or the difference in overall diameter is too small to make a difference on your car.
Or my gearbox needs tuning up
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| 11th Sep 2020 7:26 am |
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RRSTDV8
Member Since: 07 Apr 2014
Location: Here
Posts: 13493
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Strange that the different tyres would have an impact - 255/50/19 is a standard size on the equivalent RRS which used basically the same running gear including the same gearbox. Visiting from rrsport.co.uk
2012 RRS SDV6
2008 RRS TDV8
"When you fire that first shot, no matter how right you feel, you have no idea who's going to die! You don't know who's children are going to scream and burn. How many hearts will be broken. How many lives shattered. How much blood will spill, until everybody does what they were always going to have to do from the very beginning: SIT DOWN AND TALK!"
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| 11th Sep 2020 7:29 am |
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aquandes
Member Since: 11 Feb 2020
Location: Marin Epagnier
Posts: 10
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No clue.
Though the gearbox adjusted to my driving style but not the tyre size
I bought it in march this year
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| 11th Sep 2020 8:12 am |
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Motolab
Member Since: 18 Oct 2019
Location: Sleen
Posts: 1820
 
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the ECU in the Autobox is addaptive and builds up driving data... that is style AND resistance/circumstances..
you can reset the ECU back to standard, so it will addapt itself to the new condiction, tyre/wheel sizes that is... very/more quickly..
i did mount different size tyres 2 weeks ago... the autobox was also struggling with the new size...
i did want to reset the ECU with my IID, but did not have the time before a 1800km trip towards the Chech republic.. but after this trip, it adapted itself... drives smooth and without exessive shifting behaviour now... Best regards
Harold
Always looking for Pre '55's & Pre war British Motorcycles! knowing or having one for sale? PM please. I visit the UK 6 times a year
Ps. I edit my texts quite often, english is not my native language, so I will edit My “typo’s” etc. 
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| 11th Sep 2020 8:26 am |
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smoke4k
Member Since: 21 Oct 2020
Location: Chippenham
Posts: 50
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Do you need a IID to reset the auto box? I know in the BMW ZF boxes there is a procedure that you can do to reset the box adaptations without coding.
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| 8th Nov 2020 7:30 pm |
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