I know, I should do a search. I did, and read post for 45 minutes to an hour and still didn't get a straight answer.

How can I tell if I have a 10 bolt or 9 Bolt rear end?

I have a 89 IROC, 350 with rear disk brakes and 2.77 gears, I know this from the RPO codes in the console.

If I have a 9 bolt, I will have a 2 series carrier, correct? If I do, what are my options as far as gear upgrades?

I would like to put a 3.73 gear in it.

Thanks

Look under the car and count the bolts on the rear cover. 9 or 10 bolts on the cover.

A quick and simple check is to just look at the rear cover. A smooth cover is a 10 bolt. A cover with a rubber plug in it is a 9 bolt. If it has a metal plug in it, it's a rare Dana 44.

Don't always believe the RPO codes unless you're the original owner. Anything could have been changed in the car's life including the center console with the RPO sticker (VIN number matches the car?).

Since you say the RPO code is for 2.77 gears then your car originally came with a 9 bolt as that isn't a gear option for the 10 bolt. The 350 IROC cars got 2.77 gears unless they have the G92 option. Then they got the slightly better 3.27 gears. If you have 2.77 gears, you have a 2 series carrier which is good for a paperweight. You'll need a 3 series carrier to use deeper gears. Options are available but not cheap as all 9 bolt gears and major components such as a posi etc come from Australia. Your best option is to go to www.9bolt.com

That wasn't what I wanted to hear. I may be stuck with the 2.77's then.

I've seen some 3 series differetnials on here for sale and on ebay. You can get one of them and install the 3.73 gears. You can also get them new, but they are expensive.

I've got a 9 bolt with 3.27 gears and a 3 series posi for sale.

Check summitracing.com I was on the site and found ring and pinion options for both carrier sizes.