Hey guys I’ve recently changed my tyres from 205/40/R18 to 225/40/R18 people online are saying I should change the sizing in asbuilt but surely this hasn’t changed the height of the tyre only the width?
Could someone please advise 
I was told that the width of the tire was a percentage of the height. So basiclly not all 40 sidewalls are the same and will change based on the width of the tire.
That makes sense, I couldn’t figure out how changing a width affected a height/speedo 
, any one know off hand what asbuilt config I should update?
The xxx/40 means 40% of XXX, which gives you how “tall” the tire is.
So you changed from 82 mm to 90 mm.
Have you checked this? Sync 3 APIM Database - CyanLabs - Vehicle Specific Information
There’s a value for the circumference…
Makes absolute sense thanks 
Presumably it needs to be set on a few modules
Check this site for calculations Tire Size Calculator
About where it should be changed, that’s a good question…
I’ve no idea…
Not sure if the IPC has a value for that…
Usually the PCM and TCM…will need a TCM relearn after changing the size in the BCM.
Thanks, but i don’t have a forscan license, nor a valid trial license, any ideas how it’s done on UCDS?
Look in UCDS under the Fiesta 2017> tab on the home page. You could change the tire size manually as it describes in the FORScan tutorial and then just reset the TCM.
No tire size procedure only tpms, I’ll try updating manually once my laptop decides to work or I take my work laptop home tomorrow
So the tyre size set in my car currently is no where near correct it was 1894mm so I bet it was never even configured correctly to 205/40/R18 from factory lol
Apparently for some or maybe all cars you need to times the circumference by 0.967 now my speedo is about 2-3mph above GPS like it was before and as I believe UK law requires
UK law requires the speedometer not to under read and to be within 10% accuracy. Manufacturers usually set the speedometer calibration to be right at 30 MPH and progressively more optimistic as one goes faster. I have to achieve an indicated 110 to be doing 99 in my MGF which is supposedly capable of 130 which is the most optimistic device I have ever seen. I use a Garmin all the time instead!
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