AlfaOBD Tire Size/Speedo Conversion Calculation (mm)

[Ready Go]

This may be helpful for anyone using AlfaOBD to correct their speedometer after changing tire sizes from OEM.

The Conversion number to use is...
77.1786833856

Example:
(TireHeight x Conv# = AlfaOBDmm)
31.9" x 77.1786833856 = 2462.00
33.0" x 77.1786833856 = 2546.90
34.1" x 77.1786833856 = 2631.79
34.6" x 77.1786833856 = 2670.38

My understanding is that 2700mm is the approximate limit number for AlfaOBD programming..above which causes problems for OEM "Park Sense" to not function..that equates to a 34.98" tire size. I imagine most tires are fine therefore, other than anyone swapping up to 37s (??).

[I've worked the math on this a couple ways, and it seems to hold true..but not all tire manufacturers hold to the exact heights, etc, so some small adjustments to the mm input number could be needed yet between different brands]

 

[boogielander]

wow you actually took the time to find the conversion number lol
i just set it to Toyo 35x12.5R18 and call it a day lol

 

[Ready Go]

I haven't switched from my OEM tires yet, so haven't checked the AlfaOBD parameters yet for tire size adjustments..I didn't know there's actual tires listed in there, I thought you had to do the "mm" conversion to recalibrate the speedometer.
Do you know if there's a benefit to one way over the other for setting it?

 

[HSKR R/T]

I haven't switched from my OEM tires yet, so haven't checked the AlfaOBD parameters yet for tire size adjustments..I didn't know there's actual tires listed in there, I thought you had to do the "mm" conversion to recalibrate the speedometer.
Do you know if there's a benefit to one way over the other for setting it?

No need to do the mm conversion. And FWIW, my truck was due with parking sensors using my TazerDT to set tire to 34.9" until I received a software update when I took the truck in for a service 4wd light. Since then, I have lost my parking sensors, even set to 33.5". Not sure why this software update screwed everything up.

 

[Ready Go]

No need to do the mm conversion. And FWIW, my truck was due with parking sensors using my TazerDT to set tire to 34.9" until I received a software update when I took the truck in for a service 4wd light. Since then, I have lost my parking sensors, even set to 33.5". Not sure why this software update screwed everything up.

What was the software update for?
Your parking sensors aren't working at all now? That sucks for sure!

 

[HSKR R/T]

What was the software update for?
Your parking sensors aren't working at all now? That sucks for sure!

Supposedly they only updated the DTCM firmware as per a TSB that deals with an intermittent service 4wd light. It didn't fix the issue,but ever since then my parking sensors have not worked. I took it back to dealer and they "recalibrated" the parking sensors. When I got the truck back, basically, all they did was reset the BCM back to factory settings, erasing everything I had changed with alfaObd and my TazerDT. The parking sensors were working. Took me a few days to realize my speedometer wasn't correct. Checked, and it was set to factory size. As soon as I changed it back to the 34.9 for my 35" tires the parking sensors stopped working again. And it's definitely due to tire size. When I first start truck, they work for a few seconds until all the computers "boot" then they just say unavailable when you try and push the button. No warning lights or anything. Before the DTCM update, I had zero issues with my parking sensors, even with tire size set to 34.9.

 

[Ready Go]

Supposedly they only updated the DTCM firmware as per a TSB that deals with an intermittent service 4wd light. It didn't fix the issue,but ever since then my parking sensors have not worked. I took it back to dealer and they "recalibrated" the parking sensors. When I got the truck back, basically, all they did was reset the BCM back to factory settings, erasing everything I had changed with alfaObd and my TazerDT. The parking sensors were working. Took me a few days to realize my speedometer wasn't correct. Checked, and it was set to factory size. As soon as I changed it back to the 34.9 for my 35" tires the parking sensors stopped working again. And it's definitely due to tire size. When I first start truck, they work for a few seconds until all the computers "boot" then they just say unavailable when you try and push the button. No warning lights or anything. Before the DTCM update, I had zero issues with my parking sensors, even with tire size set to 34.9.

Sorry to hwar of the issues!

I've been very hesitant to take my 2024 in for the two recalls because I'm having no problems..and don't want any!!

 

[boogielander]

Sorry to hwar of the issues!

I've been very hesitant to take my 2024 in for the two recalls because I'm having no problems..and don't want any!!

that's exactly how i feel too. my truck hasn't seen the inside of dealership since it got dropped off at my house in Nov 2022 and I've been getting the ABS recall notice but just ignoring them lol i fear if they do the ABS recall something is gonna stop working.

 

[boogielander]

I haven't switched from my OEM tires yet, so haven't checked the AlfaOBD parameters yet for tire size adjustments..I didn't know there's actual tires listed in there, I thought you had to do the "mm" conversion to recalibrate the speedometer.
Do you know if there's a benefit to one way over the other for setting it?

oh yeah they have a list of actual tires listed there, but not very complete.
for instance, I run 35x12.5r18 Falken Wildpeak AT3W but the closest thing being AT tires and 35x12.5R18 I could find is Toyo Open Country AT 35x12.5R18.

i'm not sure if there's benefit to do it one way over the other, other than making it a quicker process (the way I did it). I'm a few MM off compared to the actual Wildpeak AT3W size when using Toyo option, but it's been fine since mid 2023.

 

[Ready Go]

that's exactly how i feel too. my truck hasn't seen the inside of dealership since it got dropped off at my house in Nov 2022 and I've been getting the ABS recall notice but just ignoring them lol i fear if they do the ABS recall something is gonna stop working.

Yep, I don't think I'm gonna take my in..unless absolutely necessary! I hate letting anyone mess with my vehicles.

 

[boogielander]

Yep, I don't think I'm gonna take my in..unless absolutely necessary! I hate letting anyone mess with my vehicles.

only time i'll get it in for the recall is when i'm ready to put the truck up for sale, because at that point I won't care of my speedo is off or whatnot. in some states you can't register a vehicle if it has recalls not done yet iirc.

 

[Ready Go]

only time i'll get it in for the recall is when i'm ready to put the truck up for sale, because at that point I won't care of my speedo is off or whatnot. in some states you can't register a vehicle if it has recalls not done yet iirc.

True enough

 

[BlueHemi1500]

This may be helpful for anyone using AlfaOBD to correct their speedometer after changing tire sizes from OEM.

The Conversion number to use is...
77.1786833856

Example:
(TireHeight x Conv# = AlfaOBDmm)
31.9" x 77.1786833856 = 2462.00
33.0" x 77.1786833856 = 2546.90
34.1" x 77.1786833856 = 2631.79
34.6" x 77.1786833856 = 2670.38

My understanding is that 2700mm is the approximate limit number for AlfaOBD programming..above which causes problems for OEM "Park Sense" to not function..that equates to a 34.98" tire size. I imagine most tires are fine therefore, other than anyone swapping up to 37s (??).

[I've worked the math on this a couple ways, and it seems to hold true..but not all tire manufacturers hold to the exact heights, etc, so some small adjustments to the mm input number could be needed yet between different brands]

Thanks for the info.
I changed to 275/60/R20 vs the factory 275/55/R20 size. Just a bit taller to get rid of the low-profile rubber band style and give my rims a bit more protection from curb rash.
I had to just do the trial-and-error method. I got close and I know there is an error just not sure how bad/off I am.
So, I think I will need to make an adjustment to your calculated numbers. I didn't know you could enter decimals.
I have to go and see what I entered for my tire size, but just not motivated to be in the cold to hook up the bypass cable and get cold and wet while doing the circus trick to get the bypass cable installed. I will take a look when it's a bit warmer/drier to do the circus tricks.

BlueHemi1500

 

[HSKR R/T]

Thanks for the info.
I changed to 275/60/R20 vs the factory 275/55/R20 size. Just a bit taller to get rid of the low-profile rubber band style and give my rims a bit more protection from curb rash.
I had to just do the trial-and-error method. I got close and I know there is an error just not sure how bad/off I am.
So, I think I will need to make an adjustment to your calculated numbers. I didn't know you could enter decimals.
I have to go and see what I entered for my tire size, but just not motivated to be in the cold to hook up the bypass cable and get cold and wet while doing the circus trick to get the bypass cable installed. I will take a look when it's a bit warmer/drier to do the circus tricks.

BlueHemi1500

You could just leave the bypass connected all the time. You only gained 1/2" of sidewall.

 

[Ready Go]

To be precise...0.55"