P1635 in TCM or PCM after tire size change

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Joined: Fri Jun 14, 2019 6:59 pm
Vehicle: Ranger Wildtrak, Diesel 3.2L Duratorq/200HP, 2018

Re: P1635 in TCM or PCM after tire size change

Post by josephj22 » Fri Jun 28, 2019 1:47 am

I have the 2018 ford ranger wildtrak with 3.2L diesel engine, yesterday I tried to update the hex value of my tire size as below:
- Original size 265/60R18 BCM value 934
- New tire size 285/70R17 updated BCM value 9DE

Result P1635, cannot clear even after attempting PCM relearn.

Then I followed the advise here and checked the live ID of tiresize Rev/mile and found the following:
- on original tire size: 683 Rev/mile equivalent to 30.5" tire
- on updated tire size with p1635 code tire size live value: 664 Rev/mile, which means that this is the minimum acceptable Rev/mile for TCM.

I changed again the BCM value to 977 which is equivalent to a tire size of 31.5" to match with 664 Rev/mile now p1635 code cleared.

Road test results:

- on original tires cruise control set at 117Km/h GPS speed 113Km/h
- on new tires (32.7") without BCM update: 110Km/h cruise control, GPS speed 113Km/h
- on new tires with BCM value updated to maximum allowable tire size (minimum allowable rev/mile of 664 for 31.5" tire) now cruise control and GPS speed are exact match and sometimes only 1Km/h difference at higher speeds.

Now I left out wondering what strategy the manufacturer uses to calculate speed and odometer reading? If on original settings and tires, the speedometer was not accurate, and now with upgraded tire size and updated BCM tire size for a smaller tire then actual speedometer is accurate. Does that mean that from factory they keep on purpose speedometer readings higher than GPS? And if this is the case what about odometer readings?

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