Unfortunately, the folks that compile that data doing it for profit, and generate that profit by selling the books with that info in it.
But, the first thing you need to know is what vehicle the axle came from. Let us know that and we'll tell you what the rim width is.
If you don't know the year make and model, then the other alternative is to try to take an educated guess based on the bolt pattern and possibly some other hints. You've already told us the number of lugs, and is 2 1/2" the distance between these lugs?
Any other hints?