chevytruckman2004

I want to change the differential oil in my front and rear differentials but I need to know what axles I have so I know how much gear oil to buy. How can I tell?

ToooTall1

Look in your glove box for a sticker that has the RPO codes. Those are the code the manufacture uses to identify what they put on the vehicle. Then go online and search for GM RPO code list and look up for what was installed in your vehicle. A G80 limited slip rear differential requires synthetic oil like lucas or royal purple or any good brand. Your front is not a locker and only requires regular gear oil. If you don't have a G80 then you more than likely have a non locker required and good 80-90w gear oil. If it was me I would put a good synthetic in both anyway. Both axles take close to 2 qts each. Axle ratios are like GT-4 (3.73) GT-5 (4.10) etc.

chevytruckman2004

Ok thanks, I was going to put a good synthetic in them anyway. I'm thinking royal purple. Should I flush the axles out with anything?

country_09

Drain and fill will suffice

chevytruckman2004

Ok thanks again.

chevytruckman2004

So I found out my rearend is a 3.73 Positraction/Limited Slip. Do I need to add a limited slip additive since I am going to use Royal Purple 80w90?

ToooTall1

Since you have the G-80 rear axle I would use the Royal Purple synthetic oil with no additives.

chevytruckman2004

Ok thanks. I was just wondering because I've heard that some you have to add a limited-slip additive.

FF Dave

Hi Folks,

I have a 1974 Chevrolet P-30 Step Van that I am trying to identify the axle ratio of.

To the best of my knowledge it started life in the Air Force, was sold to a Chorpus Christi school district before I bought it at auction.

So far, I've learned that it is a 1-ton, 14 bolt with dual rear tires.

* Is there a stamped number or code on the axle that would help me identify whats inside?

Unfortunately, there is no RPO code sticker in the glove box (it doesn't have a glove box), and doesn't even have a Vin Tag on it that I can find... It has had a hard life, but is structurally intact.

The big issue with it, is that there was an engine / transmission swap to a V8 / auto trans, and the drive shaft parking brake was removed & never re-installed.

The axle backing plates do not have emergency brake cable connections, so I can't get it street legal.

I do have an old 1992 C3500 that I can swap out the rear axle with, but would like to keep the same ratio... the old C3500 got around 9mpg.

Thanks, Dave

FF Dave

Hi Folks,

Got lucky, went to the Chevrolet dealership in Georgetown TX with the VIN's from the titles for both my P30 and axle donor C3500.

The Georgetown Chevrolet parts guys were great, a lot better and much more supportive than the Chevy dealership I've been going to in Austin TX.

They advised the axle in my donor vehicle (or at least what was originally installed) was a 3.73 gear ratio.

No luck with the VIN from my 1974 P30, it probably would be a 3.73 as well - can't see having a smaller gear installed on a P30 with an inline 6 cylinder, even if was the bigger 292ci engine originally.

Mighty good news indeed.

Probably be a month or so before I can get my P30 down to the ranch and do the axle swap.

Will update as work progresses.

Was kinda surprised that there were no replys, but guess since I posted on an old thread that nobody looked at it.

Dave