Tire size change alfaOBD

[drprymer]

Damn 11 mph is a huge difference, shouldn’t have been that far off. When I switched from stock to 35’s I was off 2 mph.
Anyway, Alfa doesn’t change shift points but having the speedo accurate will.

Yeah I couldnt believe it was that far off either but its dead on now so im happy

* Just an update truck does shift a lot smoother now. I changed this on my lunch break so I didnt get to test it out till the drive home.

 

[Keith Hall]

So I did this a couple weeks ago changing from 265/70R17 to the preset Goodyear SR-A 275/60R20 and the speedometer is accurate now but I never got any CELs? I went through the ABS initialization anyway. I feel like maybe I missed something because I didn't get CELs or other error?

 

[WTM75]

So I did this a couple weeks ago changing from 265/70R17 to the preset Goodyear SR-A 275/60R20 and the speedometer is accurate now but I never got any CELs? I went through the ABS initialization anyway. I feel like maybe I missed something because I didn't get CELs or other error?

I didn’t get any either, but like you, I did the ABS. This was a couple months ago now and everything has been working fine.

 

[HTXRamExp]

I never got any codes either, but I also could never complete the Clearing Rolls step as it would error out. Turns out my 275/65r20 tires measure ~33" mounted so my speedo was pretty well spot on regardless.


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[TMRoper1669]

I can not get this to work at all, I’ve followed step by step and every time I get Static ECU done and go to clear codes it will not clear and codes in the ABS, I get the same code. Now I’ve got the 3 dashes, no ABS and No power steering, any ideas?

 

[1500ram12]

I can not get this to work at all, I’ve followed step by step and every time I get Static ECU done and go to clear codes it will not clear and codes in the ABS, I get the same code. Now I’ve got the 3 dashes, no ABS and No power steering, any ideas?

Connect to the bcm and ecu and clear any codes then check the abs again

Are you using the android or windows version?


2018 2500 6.4L

 

[keepemrollin14]

I can not get this to work at all, I’ve followed step by step and every time I get Static ECU done and go to clear codes it will not clear and codes in the ABS, I get the same code. Now I’ve got the 3 dashes, no ABS and No power steering, any ideas?

Did you clear rolls?


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[TMRoper1669]

I cleared all codes In the ECU & BCM but it wouldn’t let me clear the ABS, and since i can’t clear the ABS it won’t let me clear rolls, but here is the weird thing, last night when I had not power steering, abs and all sorts of lights, I came home pulled the battery cables, turned on the headlights and let it sit for about an hour, hooked it up and took it for a drive and speedo is dead on. Who the hell knows but at least everything is back to normal and the speedo is on. LOL Computers. FYI I’m using the android version.

 

[Jack187]

Nice write up and support in this thread.

 

[Jack187]

I did my speedometer yesterday. Took me a little bit to figure out the minor differences.
I took my time, not wanting to hose anything. Probably about 1.5 hours total including installing the connector software and alfaOBD, make changes to the tire size, test the tire size change, alter it (I was off by 3.5% on first try), and retest. Running 37" tires, it is now spot on with no error indicators. I also noticed the shifting seems more response. I assume it effectively altered the shift points based on the change. Thanks again to the people that participated in this thread. The information and responses were what I needed to take the leap.

 

[RockyAEV]

Ok can I get a little help on the tire size part....forgive me if someone else has asked this I searched first before asking...

So I need to adjust my tire size for the same reasons as everyone else. I feel confident with the instructions above I can do this. My only issue is the tire size part, so in the drop down the tires I installed is not an option so I need to put in the MM size, but am so lost on what to measure to get the correct numbers it’s not even funny!

Don’t kill me I went from the stock tradesmen 275/70 R18 to the stock Laramie rims at 275/65 R20 (wanted to retain the smooth towing that the stock height and width gives)

But at any rate could someone please help me figure this out...I’m probably either not seeing the correct drop down or just being a big dummy and will slap myself when y’all answer lol thanks ahead of time for any help.


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[G. Mcpherson]

Measure your tires mounted with a level on top of the tire. Convert Diameter to Circumference in mm and go from there. Use a GPS to get your Speedo dialed in. Might take a bit of trying.

 

[RockyAEV]

So just take a level, set it on top of the tire and measure from the ground to the level? That’s it? Well damn I was thinking you had to convert the width and the height and everything lol that should be simple enough! Thanks for the info


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[1500ram12]

So just take a level, set it on top of the tire and measure from the ground to the level? That’s it? Well damn I was thinking you had to convert the width and the height and everything lol that should be simple enough! Thanks for the info


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You can use this once you get your actual tire height

https://tiresize.com/calculator/

Or use the tire size listed on the tire and adjust from there.


2018 2500 6.4L Tradesman

 

[RockyAEV]

Ok so I put it in that calculator and converted to MM,

Diameter-866
Width-275
Sidewall-178
Circumference-2718
Revs/km-368

I’m guessing I need to use the 2718 for my numerical input? I’m just terrified of doing something wrong and getting to pay the dealer a stupid amount of money to fix my ignorance lol! Thanks again for all the help.


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[1500ram12]

Ok so I put it in that calculator and converted to MM,

Diameter-866
Width-275
Sidewall-178
Circumference-2718
Revs/km-368

I’m guessing I need to use the 2718 for my numerical input? I’m just terrified of doing something wrong and getting to pay the dealer a stupid amount of money to fix my ignorance lol! Thanks again for all the help.


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Yes you'll want the 2718. You may end up having to try a couple times to get the speed correct. With my 35's I ended up having to use something like a 33.7" tire size to get my speedo close. I'm still off by 1mph


2018 2500 6.4L Tradesman

 

[RockyAEV]

Awesome thanks for the input. I’m only 3.5 mph off at 70 but my biggest concern is mileage both fuel accuracy and for services.


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[RockyAEV]

Ok so here are my results that I have found. I made several changes trying to get it as close as possible and may try again later..just got tired of messing with it.

When I put in 2718 MM my speedometer said 65 but my actual speed was 68

When I put in 2731mm my speedometer said 65 but my actual was 63

When I put in 2725mm my speedometer said 65 but my actual was 66-66.5

When I put in 2720mm my speedometer said 65 but my actual was 64. When at 75 on the speedometer my actual speed is 73.5.......I’m afraid this is as close as it’s gonna get unless someone else has better plan!

So for a tire that is 275/65 R 20 the best metric conversion that’s closest in speed is 2720 mm


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[1500ram12]

Ok so here are my results that I have found. I made several changes trying to get it as close as possible and may try again later..just got tired of messing with it.

When I put in 2718 MM my speedometer said 65 but my actual speed was 68

When I put in 2731mm my speedometer said 65 but my actual was 63

When I put in 2725mm my speedometer said 65 but my actual was 66-66.5

When I put in 2720mm my speedometer said 65 but my actual was 64. When at 75 on the speedometer my actual speed is 73.5.......I’m afraid this is as close as it’s gonna get unless someone else has better plan!

So for a tire that is 275/65 R 20 the best metric conversion that’s closest in speed is 2720 mm


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It's probably going to be 2722 or 2723


2018 2500 6.4L Tradesman

 

[G. Mcpherson]

I have a 34.3 tire in 295-70-18. Tire size says 2733mm. I measured and had 33.5". Went from there and ended up with 2689mm and have GPS exact at 60mph. Came up to 33.7. You wont hurt anything just keep trying.