i pulled this tag from my front differential cover. what does these numbers mean?
first line-520903973
second line- 73 610663-2

production numbers

Are those the only numbers on it?

could you elaborate? im looking for the gear ratio

See the 3973= 3.73.

Other numbers are just casting codes and date codes.
Also the single 73 prob has or had a 3. In front of it that either your missing or has eroded away.

The part number of the axle and factory date codes of where and when the axle was built. There should be another tag with the gear ratio on it.

610663-2, This is the number that would be the most useful. It is the Axle Bill of Material number. Basically it is the breakdown of what the axle is for, application, parts, ratio, diff type, etc.

Use this link to find out more info about that axle, plug in the number above and see what you find. There is all sorts of cool stuff on their website about axles and stuff, the literature/catalogs are handy for reference too.

http://www2.dana.com/expert/