Well, I did some math (my mom always said it would come in handy someday) and I came up with some equations that seem to work...at least they did for my CA.  (If you see any errors in my logic or calculations please let me know.)

I have all this in an Excel spreadsheet.  If anyone wants it, PM me with your email address and I'll send it to you.

I used the measurements from the chains for my CA (tire size: 11.2-24) in the examples below.  And I rounded Pi to 3.14159

Measurements Needed:

Chain Width (CW)

Tire Width (TW)

Tire Radius (TR); this can also be calculated if you have the Tire Circumference (TC)

My measurements

CW: 17" (Chain length was posted as 114")

TW: 11.2"

TC: 132" (As best as I was able to measure it by myself)

If radius needs to be calculated from circumference

(For reference circumference of a circle is 2*Pi*radius)

Formula

TC=2*Pi*TR

Example

132=2*Pi*TR

66=3.14159*TR

21.01=TR

Chain Width Adjustment (CWA)

Formula

CWA=(CW-TW)/2

Example

CWA=(17-11.2)/2

CWA=5.80/2

CWA=2.90

Radius of chains on tire (CR)

Formula

CR=TR-CWA

Example

CR=21.01-2.90

CR=18.11

Minimum chain length (MCL)

Formula

MCL=2*Pi*CR

Example

MCL=2*Pi*18.11

MCL=36.22*Pi

MCL=36.22*3.14159

MCL=113.78

So to make this as a single equation

Formula

MCL=2*Pi*(TR-((CW-TW)/2))

Example

MCL=2*3.14159*(21.01-((17-11.2)/2))

These equations will only tell you if the chains are long enough to go around the tire.  Some common sense still has to be used.  You need to determine if the width of the chains is appropriate for your sized tire.  For example, you’re not going to put a 10” wide set of chains on a 16” wide set of tires.

Thanks for the heads-up, Ted.  I'll check them out.  I need a couple of sets for my D17IV.

However, last fall I was just looking for a inexpensive set for my CA.  I found that most used sets were listed L x W, or by the tractor they were from.  I'll be the first to admit I don't know all that much about the Allis tractors, but all I know is Allis.  So if a set of chains was advertised to have come off, say, a Ford 8N and were 145" long x 17" wide, I had no idea if they would fit on my tractor, which may or may not have standard-sized tires. (Standard meaning common among the different manufacturers.) 

I came up with this formula to make it easier to determine if the chains are the right size.  As soon as I enter in the lenth and width numbers of the chains, I can tell if they would fit my tires.  I also have 2 different sets (sizes) of tires for my D17.  So now I'll be keeping an eye out for good deals on chains that would fit those tires.

Thanks,
Greg