tire diameter/pcm programming

From: Jeff Crook (Jeff.Crook@ecitelebn.com)
Date: Fri Oct 06 2000 - 13:32:38 EDT


Steve E. wrote
"No, the tire size is adjustable. They are quite proud of the fact that you
no longer have to replace a part! Just a setting in the PCM. However, the
range is so small there's not much point to it. They allow you to go from
a 28 inch tire to a 29 inch tire. That's it. Well at least using the revs
per mile method.
<snip>
The second way you can adjust is to enter the revs per mile for the tire in
use. The range is approximately 723-755 revs/mile."

Steve
programming is applied to anti lock brake computer, which derives speed from axle rev data input, then sends speed info to PCM. but that's minor nit=picking. <excuse me., please>
main point is
723 revs per mile = 27.89 tire diameter
755 revs per mile = 26.7135 tire diameter

5280 / 723 = 7.3029 multiply by 12(inches per foot) = 87.63485 divide by PI(3.1415) = 27.89586
5280 /755 =9.9933 x12 = 83.9205 /3.1415 =26.7135

My FSM for 2000 dak, page 22-6 shows
215/75r15 = 760 revs per mile
235/75r15 = 729
255/65r15 = 755
255/55r17 = 750
31x10.50x15 = 689

 these pre-defined sizes have to be available to be programmed in.as these are the available factory tire sizes
I would use the 760 rev programming , which comes out to 26.53 tire diameter.(pretty close to your desired 26.5 diameter)
Jeff Crook

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