PUL/MILE and speed DIVISOR understanding

[Marc Randolph]

Here is something for both the beginners that want to learn
about code and tables, and a question for those that already
understand most of it:

In the ANHT 90 L98 ECMGUY .pdf listing, there are two entries that
I'm very interested in and was wondering if anyone could help me 
understand them better before I go blindly changing my PROM and 
getting myself really confused (or worse).  Here's the details of
what I know:

Two table entries of interest:

8014:   FCB $00    ; INSTRUMENT PANEL DIVIDER ARG
8015:   FCB 230    ; PULSE/MILE ROAD SPEED CONSTANT

Code of interest:

At code offset BEF7, the 8014 value is OR'ed with whatever is in
memory location 0x32 (after 0x32 is masked with 0x0F right below
BEE9).  0x32 appears to be a word that gets clocked out of
the ECM serially (on YZ body, 0x32 contains, among other things 
I'm unsure of, a bit to enable the secondary fuel pump, and a
bit to control the shift light), I assume to the CCM for display
on the instrument panel.

At code offset BF9A, the 8015 value is read into accumulator A and
then multiplied by accumulator B (which contains a count of some
kind from the vehicle speed sensor, read from memory location 0x8D).
This is then divided by whatever is at memory location (0x8B).


Concepts of interest:

The ECM converts the vehicle speed sensor signal to 4007 pulses
per second and feeds that to devices like the CCM, ABS, and ASR.
The value at 8015, 230dec, is the internal representation of
the 4007.

My main question:

When I go to install taller gears in my car, I'd like to modify
the ECM to make up for the installation so that my speedometer
reads the correct speed.  My initial guess is that I should change
the value at 8015 so that the ECM puts out fewer than 4007 pulses
per mile.  Since everything else is expecting 4007 pulses per mile,
if I feed it a few less pulses, they will think the car is traveling
slightly slower (which is true because of the taller rear end).
Can anyone think of a reason this wouldn't work (or would do any
harm)?

Second question:

Would the correct pulse / mile value be based upon a percentage
of the original?  3.45 gears stock, 4.10 gears new, would result
in an 18.8% difference - is that the right amount to adjust the
pulses / mile?

Third question:

What in the world is the value at 8014 for?  Any ideas how much
it would divide/multiply the speed by?


Thank you, and have fun,

   Marc

-- 
  Marc Randolph     -    mrand at pobox.com    -     PGP keyID: 0x4C95994D
     If you have any info on the mid-60's car called the Bill Thomas
       Cheetah, or know anyone that might, please contact me.