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Discussion Starter · #1 ·
So some weeks ago I believe it was @sportcoupe who posted that there were some new parameters available in FORSCan regarding tire sizes and such.

Today I finally took time to mess with it and did the following settings on my 245/75 shoed van:

Changed tire size to this:
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(it doesn't allow you to specify a size though)

Changed tire circumference to the maximum available value of 2260 mm:
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(Stock tires are 2235 mm, 245/75 tires are 2431 mm, so only a little closer)

Changed speedometer bias to value 02:
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The Breakthrough Result:

I got the speedometer to read differently than it had been!
The sort of part:
While it reads differently, it is still not accurate (but closer than it was)

Here is a TorquePro screenshot with larger tires: Here is the same screenshot after the above changes:
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So, I got it to move a few mph at this speed, I still need to test other speeds and dig for more settings, but I count it as victory to get it to change at all ?

It was probably only the circumference setting that changed the speedometer, but further research is in order. ?
 

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The easier alternative, just add a GPS digital speedometer and ignore the OEM dash speedo. ;)

Semper Fi
 

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Discussion Starter · #3 ·
The easier alternative, just add a GPS digital speedometer and ignore the OEM dash speedo. ;)
Yes, hence the TorquePro app on my Android stereo. Yet many have sought after a way to adjust the speedometer/odometer to no avail.
 

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@asdrew yes it was me and thanks for the good work.

I too have 245/75 tires and it's off by 10%. I am very used to adding the first number on the indicated digital speedo to to get actual speed. i.e. 55 mph indicated is 55+5=60. Super easy.

Adjusting speedo halfway to 5% error would make it harder for a while. LOL
 

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Discussion Starter · #6 · (Edited)
As I drove it a bit today, it appears that at most speeds the speedometer needle is close enough to rely on, now I just have to get used to looking at it again, lol.
here are some examples:

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This looks very promising. Thanks for doing all the legwork, you two. I already purchased the recommended USB ODB2 reader and installed FORScan. Now I’m waiting for the van to be built. You’ve done my homework for me!
 
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Discussion Starter · #9 ·
Yes,with 245/75 it was just shy of 10% off before changing parameters. Yes, the changes correct it partially, and at lower speeds the Speedo needle is very close to GPS speed.

65mph CanBus reported speed was 5.8mph below GPS, now it is showing 2.8mph

Here's an old screenshot with 245/75 tires:

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Discussion Starter · #11 ·
I was referring to the odometer, or miles counter, not the speed. I wasn't sure if they are linked or not.
Oh, I see. Forgot the odometer is different than speedometer, lol
Haven't fooled with that so far, but expect it too is tied to rolling circumference. #challengeaccepted :geek:
 

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This is good news. I have the same 245 x 75 tire speed error. One of my concerns is that with the canbus reporting a 10% lower speed, items like shift points, traction control, stability control etc., are probably off at least to some extent.
 

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Changed speedometer bias to value 02:
View attachment 132133

. ?
Any idea what speedometer bias does in the Transit?

I googled it and came up with a weird explanation

Your speedometer reading must be within a range of plus or minus four percent off, but that's four percent over the entire range of the speedometer, and that range can then be shifted in what's called a bias.
 

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Discussion Starter · #16 ·
Any idea what speedometer bias does in the Transit?
Not sure, but I changed it fwiw. It would be great if you could enter a percent.
Later I will experiment with it more to see if it is tied to the current results or not.
 

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This is good news. I have the same 245 x 75 tire speed error. One of my concerns is that with the canbus reporting a 10% lower speed, items like shift points, traction control, stability control etc., are probably off at least to some extent.
This would be neat.

I couldn't really care less about what the speedo reads, I'm looking more to tweak my shift points.

Ideally I would be able to push it ~3-500 rpm higher, given all the torque converter failures (mine is already chattering). I'm not sure how much range is available in the tool though.
 

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Discussion Starter · #18 ·

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Discussion Starter · #20 ·
Once again I took the bait and went outside to tinker.
correct me if I'm wrong:
2235 mm (stock circumference) divided by 2431 mm (larger circumference) times 3.31 (stock ratio) = 3.043 (effective ratio now)

Lo and behold the latest FORSCan build lets me select a 3.05 final drive ratio. Perhaps transmission shift points use this data :geek: Unfortunately there is no simple scientific way to verify shift points before & after.

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