Speedometer Calibration With Larger / Bigger Tires?

[tburner]

@tburner many forum members have gone with bigger tire and wheel packages and would benefit from this information to correct speedometer error. Do you know if this is something that can be done DIY or dealership only?

It's an easy DIY. You just need FORScan set up with a compatible OBDII cable. If memory serves, you go into the BdyCM module and find something about tire circumference, then write your new circumference, save the module, then relearn the PCM and AWD modules. You might have to do the AWD module a few times.

You can calculate your circumference, but it won't be terribly accurate. I recommend following that up with a GPS-measured speed compared to your indicated speed to calculate your actual new circumference.

If I can intersect my free time with my memory to do so, I'll do a write-up with the details. But all the info you need is out there for other vehicles. The process is the same.

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[Bronclahoma]

Download the Waze nav app and it shows your gps speed.

 

[Chook]

As mentioned, any GPS app will give you a fairly accurate MPH while adjusting the tire circumference in BodyCM (as tburner mentioned). I found the manufacturer circumference (Falken Wildpeak), for this usage, to be way off, so I wouldn't rely on that, at all.

After tons of testing and tweaks, here's what I set the circumference to:

- The original factory circumference on the Pirelli Scorpion ATR 225/65/17 was set at 2202.
- The new circumference setting I'm using for the Falken Wildpeak AT3W 245/65/17 is 2232.

This will get you to within .3 to .5 mph accuracy, and less, depending on the day, which is close enough for me.

Also as tburner mentioned, you have to relearn the PCM and AWD modules.

 

[Meanderthal]

As mentioned, any GPS app will give you a fairly accurate MPH while adjusting the tire circumference in BodyCM (as tburner mentioned). I found the manufacturer circumference (Falken Wildpeak), for this usage, to be way off, so I wouldn't rely on that, at all.

After tons of testing and tweaks, here's what I set the circumference to:

- The original factory circumference on the Pirelli Scorpion ATR 225/65/17 was set at 2202.
- The new circumference setting I'm using for the Falken Wildpeak AT3W 245/65/17 is 2232.

This will get you to within .3 to .5 mph accuracy, and less, depending on the day, which is close enough for me.

Also as tburner mentioned, you have to relearn the PCM and AWD modules.

I believe the number shown in BodyCM is the circumference of the tire in millimeters. Not 100% sure but I think that is right.

 

[Chook]

I believe the number shown in BodyCM is the circumference of the tire in millimeters. Not 100% sure but I think that is right.

I think millimeter’s is correct.

 

[baztien.xplrz]

Hey all,

Hoping to get some advice in this thread with some hiccups I'm running into when attempting to update my tire size via Forscan. I've recently upgraded my stock 225/65/17 (2202mm circum) to 245/65/17 (2357mm circum) -- calculated circum via two different online tire calculators. I was able to successfully write in the new size, that's when the expected AWD error + orange service wrench came up on my dashboard and DTC PCM alerts came up on Forscan.

I followed the directions noted here (https://forscan.org/forum/viewtopic.php?f=16&t=15994&sid=2c2e7131453abf2d477a5b00351ca3f8)
to relearn the PCM vehicle data, however I noticed AWD module does not show/exist under my list of DTCs.

I am using FORscan v2.3.55 and Vgate vLinker FS OBD2 USB Adapter to try and make these changes.

I am wondering if I need to use a previous version of Forscan (perhaps v2.3.4 ?) or a different OBD adapter? or if AWD module is under a different acronmy?

Appreciate any help I could get here to get over these Forscan hurdles ?

 

[wessermgm]

I would think that Ford dealers would desire to get the speedo recalibrated because whatever % the speedo is off, the odometer will read less by the same %. So if you are looking a 3.5% miscalculation for larger tires (about the difference at 65 mph for a switch from 225/65/17 to 245/65/17, that would correlate 3.5% odometer error in less mileage being recorded. Over the course of the manufacturer 36,000 mile warranty, that would mean your warranty would actually expire when your odometer reads 34,740. Question is would the dealership catch this or care? Probably not.

 

[Mark S.]

I would think that Ford dealers would desire to get the speedo recalibrated because whatever % the speedo is off, the odometer will read less by the same %. So if you are looking a 3.5% miscalculation for larger tires (about the difference at 65 mph for a switch from 225/65/17 to 245/65/17, that would correlate 3.5% odometer error in less mileage being recorded. Over the course of the manufacturer 36,000 mile warranty, that would mean your warranty would actually expire when your odometer reads 34,740. Question is would the dealership catch this or care? Probably not.

I think Ford would rather you install tires with the same diameter as OEM tires.

 

[wessermgm]

I think Ford would rather you install tires with the same diameter as OEM tires.

I'm sure they would, but they also are fully aware that many choose to upgrade (including Ford from 225/65/17 to 235/65/17). I would guess that if you did increase the diameter they would prefer you recalibrate.

 

[wessermgm]

I'm sure they would, but they also are fully aware that many choose to upgrade (including Ford from 225/65/17 to 235/65/17). I would guess that if you did increase the diameter they would prefer you recalibrate.

And yes, I know if you order the 235/65/17 from factory the speedo is calibrated for that. I was merely pointing out that Ford is aware of a desire for larger tires.

 

[Mark S.]

I'm sure they would, but they also are fully aware that many choose to upgrade (including Ford from 225/65/17 to 235/65/17). I would guess that if you did increase the diameter they would prefer you recalibrate.

In some legal circles making something possible is the same as encouraging it. Ford does not want to be seen as encouraging modifications that increase wear on suspension components and adversely affect road handling. Ford has no obligation to help owners modify their vehicles in ways that may be interpreted legally as making them more dangerous to drive.

 

[tburner]

Hey all,

Hoping to get some advice in this thread with some hiccups I'm running into when attempting to update my tire size via Forscan. I've recently upgraded my stock 225/65/17 (2202mm circum) to 245/65/17 (2357mm circum) -- calculated circum via two different online tire calculators. I was able to successfully write in the new size, that's when the expected AWD error + orange service wrench came up on my dashboard and DTC PCM alerts came up on Forscan.

I followed the directions noted here (https://forscan.org/forum/viewtopic.php?f=16&t=15994&sid=2c2e7131453abf2d477a5b00351ca3f8)
to relearn the PCM vehicle data, however I noticed AWD module does not show/exist under my list of DTCs.

I am using FORscan v2.3.55 and Vgate vLinker FS OBD2 USB Adapter to try and make these changes.

I am wondering if I need to use a previous version of Forscan (perhaps v2.3.4 ?) or a different OBD adapter? or if AWD module is under a different acronmy?

Appreciate any help I could get here to get over these Forscan hurdles ?

Your AWD module should appear in your DTC list (among a ton of other modules). I would recommend deleting your vehicle profile and making a fresh one.

The relearn functions will be on the "Configuration and Programming" screen. Referencing the below image, click the button boxed in red. Then you'll want to run the functions boxed in orange.

Then you can reset your DTCs. After resetting them, check for DTCs again to confirm they cleared. The AWD module DTC sometimes wouldn't stay cleared for me. After relearning the AWD module (maybe a few times) it would clear up.

On a related note, I have 245/65R17 Falken Wildpeak AT3Ws and my circumference is set to 2242 after calibrating against GPS. My speedometer is within about 1.5% of GPS indicated speed (<1 MPH delta at 65 MPH).

 

[RSH]

First of all you need to the accuracy of your speedometer, most speedometers are not that accurate to begin with. Going from a 225 65 17 tire tto a 245 75 17 tire size, the difference in speed due to tire diameter would be 2 mph. ,
I have 245 65 17's on my Badlands, I'm not bothered by the speed error, and truthfully you pretty much forget about it and just go about your business.

 

[tburner]

First of all you need to the accuracy of your speedometer, most speedometers are not that accurate to begin with.

Modern speedometers are plenty accurate as long as they know the actual circumference of the wheels they're measuring. You'll experience variation as the temperature of your tires changes (ideal gas law), but I speculate temperature changes will have a greater impact on pressure than volume, given the steel cables in the tire. So I imagine it's reasonable to say you're calibrating your speedometer for a given tire pressure, and that can be within +/-1 MPH at typical highway speeds. Though again, I'm just speculating. I haven't studied tires. But personal experience seems to support my theory.

Going from a 225 65 17 tire tto a 245 75 17 tire size, the difference in speed due to tire diameter would be 2 mph.

Nominally, that's roughly a 10% increase in circumference. So you're correct at 20 MPH. At 65 MPH that becomes a difference of 6.5 MPH.

 

[baztien.xplrz]

Your AWD module should appear in your DTC list (among a ton of other modules). I would recommend deleting your vehicle profile and making a fresh one.

The relearn functions will be on the "Configuration and Programming" screen. Referencing the below image, click the button boxed in red. Then you'll want to run the functions boxed in orange.

Then you can reset your DTCs. After resetting them, check for DTCs again to confirm they cleared. The AWD module DTC sometimes wouldn't stay cleared for me. After relearning the AWD module (maybe a few times) it would clear up.

On a related note, I have 245/65R17 Falken Wildpeak AT3Ws and my circumference is set to 2242 after calibrating against GPS. My speedometer is within about 1.5% of GPS indicated speed (<1 MPH delta at 65 MPH).

Thank you for the advice and input! will definitely try this out once I get some downtime