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Discussion Starter #1
Checked the PSOM constant in my truck and it's set at 9.67 and it says I have "6" as the lockout number. Steve83 said that's what is should be for the stock 235/75R15's. Life is good! Plugged the formula's in a spreadsheet but can't come up with that constant(9.67). Life is bad.

I've checked and rechecked and verified my "circumference" number and it's within .03 of 3 different online calculators(Tire Rack etc.). My Revs Per Mile is also off by a bunch. I'm going to be replacing the tires and probably put the stockers back on. If I get a deal on a different size tire, I might grab those. I want to be able to re-program the PSOM with the correct constant. Can't do that if I can't do the "ciphering". Is their a reliable online constant calculator? My number is 2 far off and the PSOM increments are .01 and my guess is that I need to be within that tolerance. How close is close enough?


My calculated numbers:
Width 235
Aspect 75
Rim 15
Tire Height 13.8780
Total Height 28.8780
Circumference 90.7227
Revs Per Mile 698.1104 = 63,360/90.72
Coversion Con 9.43 = (698.11 x 108) / 8000

Willy.
 

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28 Posts
Discussion Starter #3
Used PI in calculating the circumference of the tire. Where I come up empty is in calculating the Revs Per Mile. Any other ideas?
 

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Engineer
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13,163 Posts
What's the problem... it should be set at 9.43 and it is at 9.67. You need to reprogram it
 

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Former owner of Shadofax
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17,025 Posts
Are your tires new or worn? Big differance in my MTs .
With an online calculator that would matter, but he's doing circumference off the dimensions of his actual measurements it looks to me.
 

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Premium Member
1995 XLT SAS w D44 and D60 rear
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3,513 Posts
With an online calculator that would matter, but he's doing circumference off the dimensions of his actual measurements it looks to me.

That is why it would be different from the "stock" 9.67 would not take a lot of wear to account for .24 differance.
 
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