I can see that it is about time for a separate thread about van electronics, module programming and CAN bus info....
Changing something like the tire size is fundamentally a pretty simple concept... There is a specific location in the ECU's memory that holds some numeric value for tire size, represented in revs/mile. Hand held tuners, software packages like AE and many others read the contents of the memory, convert those contents to human readable format and then allow you to manipulate the value of certain parameters and reprogram the memory. You can communicate with modules and read the data out of the memory without any fancy user interface style software packages but you'd better know what you're looking for because it isn't conveniently formatted for human consumption. Those purpose built hand-helds and fancy interface software have done the research and spent the time to go through the data line by line and interpret the contents so that you can easily read it.
Now if you're clever and too cheap to buy a software that already does the hard work for you, there are ways to figure out which memory locations are the ones you want and there are ways to attempt to convert parts of the data into something that might be readable. In a new thread, we can toss around ideas for how to isolate the information we want to find...but in this thread, I'm only going to concentrate on what it takes to get the Cummins running in my van. I will need to manipulate some values in the modules in the van so I do have a strong desire to find a way to do all this without spending much money..... BUT... it takes a lot of time to figure out even one or two things so for now my focus is on my specific needs in my specific application. If I learn things along the way that might help crack other mysteries wide open, I'll share my findings.
I think it is annoying to have to spend a few hundred dollars on a hand-held programmer just to change the tire size so I'm going to try to find a way to do it with open source software and cheap hardware (cheap meaning under $100). I think it can be done...Actually, I know it can be done. The companies who have done it before just want to make a profit off of it by bundling it into something you might not really need.
**edited to update terminology**