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Need some hellp with alfaobd tire size 38s

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Jaymasta

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I have a 6.5 inch zone lift with 38 nitto grapplers and have been wanting to change my tire size so my speedo and transmission were working accordingly. I got the bypass harness and obd link lx. Following the guide it says to use the tire size drop down which obviously doesnt have 38s on there or the circumference in mm. how the hell do i figure that out? About to do a long road trip and dont wanna screw anything up. Appreciate any help
 
Jaymasta said:
I have a 6.5 inch zone lift with 38 nitto grapplers and have been wanting to change my tire size so my speedo and transmission were working accordingly. I got the bypass harness and obd link lx. Following the guide it says to use the tire size drop down which obviously doesnt have 38s on there or the circumference in mm. how the hell do i figure that out? About to do a long road trip and dont wanna screw anything up. Appreciate any help
Click to expand...
If you need to work out the circumference it's a matter of multiplying your tire diameter by pi. I.E. 38"x3.14.. =119.38"
Then multiply that by 25.4 to get it in mm. =3032.25mm

But I would recommend that you actually measure the diameter of your tire to make sure you have that exact. And then work out the rest. When multiplying by pi don't just use 3.14, use the function in your calculator to make it more accurate.

Sent from my SM-N9600 using Tapatalk
 
Atty said:
If you need to work out the circumference it's a matter of multiplying your tire diameter by pi. I.E. 38"x3.14.. =119.38"
Then multiply that by 25.4 to get it in mm. =3032.25mm

But I would recommend that you actually measure the diameter of your tire to make sure you have that exact. And then work out the rest. When multiplying by pi don't just use 3.14, use the function in your calculator to make it more accurate.

Sent from my SM-N9600 using Tapatalk
Click to expand...

Ah ok yeah i just am nervous about making any mistakes before doing this road trip. i measured it with a tape measure and they were only about 37 inches but im going to do it again when its light out for a better measurement.
 
Brutal_HO said:
Also make sure you know the procedure to init ABS, clear all the faults, etc.
Click to expand...
i have the pdf of the walk through but it just didnt say how to get the mm but i guess thats just basic math.. i might wait til after i go on this road trip i dont wanna mess it up
 
Jaymasta said:
. i might wait til after i go on this road trip i dont wanna mess it up
Click to expand...
You really can't mess it up. I made 6 or7 changes in mm before I got what I wanted. Some ideas:
1 - write down any change you make with AlphaOBD, that way if you do mess up you can return to what was there before you messed up
2 - Easiest inch to mm conversion, go to google and type in xx.xin = mm. It will give you the mm.
3 - get a free speedo app for your phone. Use that to see actual speed and compare to the change you made
4 - or use your phone's GPS app to make your comparisons
It can be frustrating, but some trial and error will get you to an accurate reading.
 
John Jensen said:
You really can't mess it up. I made 6 or7 changes in mm before I got what I wanted. Some ideas:
1 - write down any change you make with AlphaOBD, that way if you do mess up you can return to what was there before you messed up
2 - Easiest inch to mm conversion, go to google and type in xx.xin = mm. It will give you the mm.
3 - get a free speedo app for your phone. Use that to see actual speed and compare to the change you made
4 - or use your phone's GPS app to make your comparisons
It can be frustrating, but some trial and error will get you to an accurate reading.
Click to expand...
appreciate the confidence lol how did you find out which body computer your truck had?
 
Jaymasta said:
appreciate the confidence lol how did you find out which body computer your truck had?
Click to expand...
Here's what I wrote back in 2016 when changing my 2016 Ecodiesel tire sizes

Edit:
Step 2 – ABS module may or may not be required. If Step 1 by itself does not work, then include step 2.
My tire size changing fum had me do it with and without Step 2. Confusing to say the least.
This was the case in 2016, Alpha has made many changes since then, Step 2 may no longer be necessary.
 

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Last edited:
Use the "my2016-2017 power net". Do not use the "2013-2017"
 
Jaymasta said:
gotcha! thank you!
Click to expand...
OOOPS! Giant error on my part. Just dawned on me, we are on a 4.5 HD site.
All the programming info I gave pertains to my 2016 1500 Ecodiesel truck.
I have not had to change my 2020 3500 HO settings, so I can't properly advise you.
Sorry, but glad I woke up to the fact we are talking apples and oranges.
 
John Jensen said:
OOOPS! Giant error on my part. Just dawned on me, we are on a 4.5 HD site.
All the programming info I gave pertains to my 2016 1500 Ecodiesel truck.
I have not had to change my 2020 3500 HO settings, so I can't properly advise you.
Sorry, but glad I woke up to the fact we are talking apples and oranges.
Click to expand...
ah ok yeah i have a 2021 2500 cummins
 
id assume id use the MY2021 PowerNet ?

not the MY2013-2017 Powernet like this guide is telling me? he was using a 2018
 
Easiest way to measure the tire.
Set your tire pressures at what you plan on running on a daily basis. It makes a difference on the actual tire size.
Mark the rear tire and ground at a spot. Roll the truck forward 1 revolution of the tire and mark the ground again.
Now measure the distance between the lines and convert to mm. Use that number to put into the computer. That's what the computer actually uses. The distance the tire travels per revolution.
Circumference.
 
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