Determining axle ratio question

[hinkv10]

I have a 87 V10 that came with a tbi 305, 700r, and np208. It now has a carbed 305, th350, and a np205. I can't find any markings or identification on the axle tubes as to what gear ratio the truck came with. I assume that it is original to the truck but I have no proof of that. The truck is rust free for a north eastern truck so it is possible the builder just wanted a stronger driveline and put the np205 and th350 in it.

I have the rpo codes or whatever they are called from inside the glovebox. Will that tell me what it came with? Any other way of telling? I don't want to open the diffs again, pita, smelly and I have good fresh fluid in there that I don't want to try and save and get back in there without making a mess. I will if I have to but its motorcycle season!

I finally got my sunpro tach installed and its revving about 2800 rpms when doing 65 on the highway. The speedo is kind of accurate from what i can tell. Thanks

ps. the engine is from a 84, transmission I'm unsure and the transfer case is 1979.

 

[hinkv10]

Of course as soon as I wrote this I found the link I was looking for

 

[hinkv10]

The pics helped because I thought I had cleaned 25 years of grime off the axle housing but there were a couple layers left.

Reads BC3 which is a 3.08 non locking.

 

[Jims86]

Thats a good ratio for the highway, but kils your mpg around town. thats close to my 3.42 on my burb with 700r4 hwy cruising.

 

[dietcokedream]

How did you find out? I'm wanting to know what ratio mine are. My front is a ten bolt that I believe came stock with my truck. I swapped in a ten bolt in the rear. that I got a screaming deal on when the 12 bolt I had took a crap. I wanna figure out if both diffs are the same ratio without draining the fluid if possible. The ten bolt came off of a 1984 4wd 1/2 ton Chevy Scottsdale and my truck is an 1985 gmc high Sierra 4wd 1/2 ton

 

[RetroC10Sport]

http://www.gmsquarebody.com/forum/showthread.php?t=4266

http://www.gmsquarebody.com/forum/showthread.php?t=4185

 

[dietcokedream]

Oh wow didn't know about these threads. Awesome. Thanks!

 

[RetroC10Sport]

Any time sir.