It came form a 89 Transam. Ring gear size is 7.75", has typical Borg Warner cone posi. Ring gear has 41 and pinion has 11 teeth. Number 'X11K8' is stumped on it and also stumped an arrow pointing to a dot within a circular pattern, and another number reads 0578 026903 (731). Any one has experience with 373 BW?

 

It's not original; a replacement.

It was fairly popular back when it was available. No particular problems that I can recall hearing about except some people thought it whined too much.

Your 85 should have come with a 3.70 9-bolt. That was the stock ratio that came in those. A pretty unusual piece for sure.

 

Some clarification. My original one, years ago was a BW Posi 3.70. It was replaced with 'standard 10 bolt 373' also years ago. As of last week I got one form 89 Transam thinking it was 370. But once cover opened, it was a 373. Even if 370 ring/pinion went bad it was fairly available even in 90s. Puzzled!!!!

 

At some point for reasons not known to us mere mortals, the 3.70 disappeared from the marketplace; but the 3.73 was widely available. People were all kinds of outraged and incensed over it for some reason. Didn't make much sense to me.

A 89 would never have come with either. The highest ratio it would have had was 3.45, and that only if it was a 5-speed w/ G92. There weren't many of those as well. So in any case, that's not original, either.