Speedometer Forscan Fix for 37 Inch Tires

omcvay

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Location
Fort Worth
Current Ride
F250 Platinum Tremor
After much research, trial and error I have my Speedo working within 1mph at 75. I’m not an expert but I can share with you what I’ve found if you’d like to do the same. I’m running 37x13.50R17 Ridge Grapplers.


It will not take your tire circumference so modifying the axle ratio to compensate by trial and error. You’re offsetting the axle ratios to compensate for the module not accepting a larger tire size.

It will only accept a few axle ratio inputs and I could not get it to take less than 3.20. If you have a larger tire than 37 it may not work for you.

Some of this idea came from other forums and as-built changes to the 17-19 people were successful with. I haven’t seen anyone do this on the 20 up but have seems tons of people attempting

Go to BdyCM Module Configuration

Set Tire Circumference mm to 2792
Set Axle Ratio to 3.2
Be sure to write between each change in settings
Now all of your service lights are on the dash so go
PCM Module initialization/relearn vehicle data
Then run
TCM Module initialization/relearn vehicle data
No lights on dash and trans shifts at normal points again.
 

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In case anyone is using this, I am dead on at 70mph with the following:

37/12.5r18 Falken Wildpeak AT3W
Axle ratio: 3.310
Circumference: 2780

I wanted to use the highest axle ratio it would accept while keeping the tire size below the DTC threshold (2792). I started by directly using the manufacturers revolutions per mile (571) to calculate circumference:

(5280/571)*12*25.4 = 2818

Then use the axle ratio fraction to get the circumference at 3.31 (edit: My truck is a diesel with 3.55 gears):

2818*(3.31/3.55) = 2628

This ended up being off by about 5.5% (indicated 52mph, actual 55mph). Multiplied by the fraction to get the circumference.

(55/52)*2628 = 2780
 
In case anyone is using this, I am dead on at 70mph with the following:

37/12.5r18 Falken Wildpeak AT3W
Axle ratio: 3.310
Circumference: 2780

I wanted to use the highest axle ratio it would accept while keeping the tire size below the DTC threshold (2792). I started by directly using the manufacturers revolutions per mile (571) to calculate circumference:

(5280/571)*12*25.4 = 2818

Then use the axle ratio fraction to get the circumference at 3.31 (edit: My truck is a diesel with 3.55 gears):

2818*(3.31/3.55) = 2628

This ended up being off by about 5.5% (indicated 52mph, actual 55mph). Multiplied by the fraction to get the circumference.

(55/52)*2628 = 2780
curious how this affects shift points since you actually have 3.55 gears
 
curious how this affects shift points since you actually have 3.55 gears
I had a similar worry. I guess it depends on how shift points are determined. If they are based on load/rpm, then the differential gearing shouldn’t matter. If they are based on speed, then I’m better off this way as the engine rpm/wheel speed are correct. So far, I haven’t had anything I would describe as an issue but, honestly, I accelerate like a grandma because my dog gets sick if I don’t 🙂.
 
After much research, trial and error I have my Speedo working within 1mph at 75. I’m not an expert but I can share with you what I’ve found if you’d like to do the same. I’m running 37x13.50R17 Ridge Grapplers.


It will not take your tire circumference so modifying the axle ratio to compensate by trial and error. You’re offsetting the axle ratios to compensate for the module not accepting a larger tire size.

It will only accept a few axle ratio inputs and I could not get it to take less than 3.20. If you have a larger tire than 37 it may not work for you.

Some of this idea came from other forums and as-built changes to the 17-19 people were successful with. I haven’t seen anyone do this on the 20 up but have seems tons of people attempting

Go to BdyCM Module Configuration

Set Tire Circumference mm to 2792
Set Axle Ratio to 3.2
Be sure to write between each change in settings
Now all of your service lights are on the dash so go
PCM Module initialization/relearn vehicle data
Then run
TCM Module initialization/relearn vehicle data
No lights on dash and trans shifts at normal points again.
I will try this. Thanks
 
Update none of the ratios or tire circumference worked on mine. Mine is a FX4 with 3.55 gears and 33's or 2644 mm. Kept throwing a CEL. Set back to stock values and cleared CEL.
 
In case anyone is using this, I am dead on at 70mph with the following:

37/12.5r18 Falken Wildpeak AT3W
Axle ratio: 3.310
Circumference: 2780

I wanted to use the highest axle ratio it would accept while keeping the tire size below the DTC threshold (2792). I started by directly using the manufacturers revolutions per mile (571) to calculate circumference:

(5280/571)*12*25.4 = 2818

Then use the axle ratio fraction to get the circumference at 3.31 (edit: My truck is a diesel with 3.55 gears):

2818*(3.31/3.55) = 2628

This ended up being off by about 5.5% (indicated 52mph, actual 55mph). Multiplied by the fraction to get the circumference.

(55/52)*2628 = 2780

This thread makes me realize just how "DUMB" I am. The brainpower on this forum amazes me!!!
 
I hadn’t seen anyone post numbers or results for a gas engine Tremor so I played with my settings today in Forscan. I’m now dead on with two different gps speedometers.

2022 F250 Tremor with 7.3L gas

New settings:
Axle ratio - 4.10
Tire circumference - 2780

Old settings:
Axle ratio - 4.30
Tire circumference - 2689

Changed the settings. Used the PCM relearn and boom. Spot on. Drove roughly 80 miles after the change and no CELs so far.

Tires I have are 37x12.5r20 Mickey Thompson Baja boss ATs
 

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i am pretty sure changing the axle ratio has 0 affect on the speedo on these trucks.
on my gas truck, my truck will not take any tire circumference greater than 2699 without storing a U042 code in the PCM. no cel or wrench lights though
I have tried every value, trick, reset, etc...
nothing works
 
I hadn’t seen anyone post numbers or results for a gas engine Tremor so I played with my settings today in Forscan. I’m now dead on with two different gps speedometers.

2022 F250 Tremor with 7.3L gas

New settings:
Axle ratio - 4.10
Tire circumference - 2780

Old settings:
Axle ratio - 4.30
Tire circumference - 2689

Changed the settings. Used the PCM relearn and boom. Spot on. Drove roughly 80 miles after the change and no CELs so far.

Tires I have are 37x12.5r20 Mickey Thompson Baja boss ATs
how about a stored U042 code in the PCM? have you checked? that code does not throw a light on my truck
 
i am pretty sure changing the axle ratio has 0 affect on the speedo on these trucks.
on my gas truck, my truck will not take any tire circumference greater than 2699 without storing a U042 code in the PCM. no cel or wrench lights though
I have tried every value, trick, reset, etc...
nothing works
When changing my gear ratio from 4.30 to 4.10 my speedo went from 3mph slow to 1.5-2mph slow. Once I changed my tire size to the new value, the speedo was spot on at highway speeds
 
how about a stored U042 code in the PCM? have you checked? that code does not throw a light on my truck
No stored code yet. I will keep an eye on it though.
 
i am pretty sure changing the axle ratio has 0 affect on the speedo on these trucks.
on my gas truck, my truck will not take any tire circumference greater than 2699 without storing a U042 code in the PCM. no cel or wrench lights though
I have tried every value, trick, reset, etc...
nothing works
Changing the axle ratio directly impacts indicated speed. The speedometer is a reflection of:

Engine RPM * transmission gear ratio * axle ratio * tire circumference

There are wheel speed sensors but they are part of the braking system and are only looking for abnormal differential speed (locking a wheel during panic braking, losing traction on ice, etc.)
 
When changing my gear ratio from 4.30 to 4.10 my speedo went from 3mph slow to 1.5-2mph slow. Once I changed my tire size to the new value, the speedo was spot on at highway speed
interesting. well, my truck has 3.73s and 37s. max tire circumference my truck will take is 2699. my speedo is still 2 mph slow.
i wonder what gear ratio I can put into forscan to "trick" the speedo into being correct?
thoughts?
Changing the axle ratio directly impacts indicated speed. The speedometer is a reflection of:

Engine RPM * transmission gear ratio * axle ratio * tire circumference

There are wheel speed sensors but they are part of the braking system and are only looking for abnormal differential speed (locking a wheel during panic braking, losing traction on ice, etc.)
hum... guess I am wrong.
 
interesting. well, my truck has 3.73s and 37s. max tire circumference my truck will take is 2699. my speedo is still 2 mph slow.
i wonder what gear ratio I can put into forscan to "trick" the speedo into being correct?
thoughts?

hum... guess I am wrong.
It depends on what speed you were going (GPS) when you were 2mph slow. Just do:

(Indicated speed / Actual speed) * axle ratio

So if it was 70, you’d have (68/70)*3.73=3.62


Edit: I can’t remember for sure if it allows you to use non-standard axle ratios. If it doesn’t, you’d need to move it down to the next lower standard ratio (from the example above it would be 3.55) and then move the tire circumference as well.

(Standard ratio / calculated ratio) * tire circumference

From above example: (3.55/3.62)*2699=2644
-So if this was required you’d use 3.55 axle ratio and 2644 tire circumference.
 
Last edited:
It depends on what speed you were going (GPS) when you were 2mph slow. Just do:

(Indicated speed / Actual speed) * axle ratio

So if it was 70, you’d have (68/70)*3.73=3.62
did you just write your value on the plain english side?
i wonder if the BCM will only take preset Ford values, like 3.31, 3.55, etc...?
 
did you just write your value on the plain english side?
i wonder if the BCM will only take preset Ford values, like 3.31, 3.55, etc...?
I made an update to my post above. I wrote my changes directly. I’ll check my spreadsheet later and see if I used a standard axle ratio or not. I haven’t made any changes using the plain English module.
 
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