Tire size change with AlfaOBD 2020 power wagon

[jabartram]

Has anyone successfully changed tire size in bcm with alpha? im looking at 35s but i want all calculations correct.

 

[delmoniCO]

Circumference in mm. I reduced the spec size of my tires by 5% to match my GPS.

Spec height x 3.1416 x 25.4

 

[jabartram]

Are there any adjustments that need to made to abs settings?

 

[delmoniCO]

I just followed the directions in the Alfaobd google drive doc. Biggest tip is to wait until the truck FULLY turns off when doing the power cycles. I waited 2 full minutes.

 

[jabartram]

Are the google docs located on this forum? i was looking but cant seem to find any?

 

[delmoniCO]

Tire Size AlphaOBD.docx

drive.google.com

 

[delmoniCO]

The rest of the common ones. You'll probably want to adjust TPMS thresholds.

AlphaOBD Directions - Google Drive

drive.google.com

 

[jabartram]

Thank you for the link. a pin wasnt required for the changes?

 

[delmoniCO]

Thank you for the link. a pin wasnt required for the changes?

No. You can ignore that step

 

[jabartram]

Circumference in mm. I reduced the spec size of my tires by 5% to match my GPS.

Spec height x 3.1416 x 25.4

I got my new tires installed, did the circumference measurement, did the changes in my bcm, my speedometer is now 1-2 miles faster than gps. how did you get your dead on?

 

[delmoniCO]

drive 70 mph on your GPS: what does your speedo show?

(speedo - GPS)/GPS = X

Reduce your circumference by X%

example: 70 mph GPS, 72 mph speedo
(72-70)/70 = 0.02857
0.02857 = 2.857%

reduce your circumference in Alfa by 2.857%

 

[jabartram]

Cool, thank you, ill check it in the morning.

 

[jabartram]

Worked like a charm, speedo is dead on!

 

[2powerwagons]

Just in case anybody with a PW puts 35X12.50R17 BAJA BOSS AT tires on stock rims , 2670mm is about right for the speedo. Wider rims might change that though.

Didn't really know where to put that info, but I figure I can't be the only one running (or considering) them on a PW.

 

[AnthonyD1978]

You should measure the diameter with the tires on the truck. Then convert to circumference and use that measurement in Alfa.

Don't base your measurement off the tire manufacture specs. Especially with the tire not under load on the truck. Width of the tire shouldn't be a factor.

 

[jadmt]

The guy who invented the aev procal told me to park on flat cement surface and measure from ground to center of hub and X2.

 

[AnthonyD1978]

The guy who invented the aev procal told me to park on flat cement surface and measure from ground to center of hub and X2.

That actually makes sense with the tire under load. Should be a more accurate measurement, but I've always done the whole diameter and it's always been spot on with GPS on first try.

The bigger the diameter you're measuring the more sense your method makes.