rear end

[86 chev guy]

I wrote down the serial number of my truck and gave that to a GM dealer who looked in their system to find out which rear end I have in my truck. It came back as a 3:42 rear end. So I orderd a new one from the auto wreckers and when it came time to replace the rear end I found out that it was not a 3:42 rear end. It is a 2:73 (I think that is what he told me) So the auto wreckers took some numbers from the glove box and determined the same thing. My question is this,,,how did GM determine from the serial number that it was a 3:42 instead of what it really is? I believe that the rear end was not swapped out before based on the brake lines and etc. etc.Any one with some insight into this? Thanks In advance to all who reply.

[75K30]

Welcome to the site:

It has been my experience that too many folks rely on the dash code, or production numbers / build codes to determine such things as gear ratios. Unless you are the original owner, this can be very un-reliable. Even by looking at the brake line (only one in the back) it can be very difficult to determine if an axle has been changed, especially if it was done several years ago. Trying to determine if the actual ring and pinion has been changed can be nearly impossible to check. This can be done without disturbing any of the lines at all.
The best method for verifying the gear ratio is to simply remove the inspection cover on the axle, and find the stamped numbers on the ring gear. The stamped numbers will verify the ratio of the gear set. The other method is to physically count the ring gear teeth, and divide that number by the number of pinion gear teeth.
Chances are, this cover has not been removed in a very long time, and the requirement for a new rear axle could very well be a result of a lack of maintenance.
The rear axle has oil that breaks down, and encounters a great deal of pressure and heat. This oil requires maintenance also.
Removing the cover for "Inspection" should be a routine part of preventative maintanance, as well as the first method for determining the actual gear ratio.

FWIW: that 2.75:1 ratio is a very high ratio, and you may consider installing a ratio of 3.42. You will pick up some free performance.
*Note* will require a gear change for the front differential if 4x4, but I do not have any data that shows any 4x4's with such a high ratio (numerically lower).

[gmctrucks]

I have heard of 4x4's with high gear ratio like in an older blazer that one can't even pull a boat correct I agree lower your gears.

[86 chev guy]

Thank you for your reply,,,If this rear end is the correct one(the 2:73) and to change over to 3:42 would require changing the front also. What if this is the wrong one and that is why there is so much play? There is a lot of movement and the mechanic said that the gears are all worn out. Replacement is required to correct this situation. Could one find out the gear ratio from the front end? This is a 4x4. I don't want to replace the rear end if it will not perform to what is wanted and/or replace the rear end that is there if it is the wrong one. If someone had replaced this rear end before with the wrong one then looking at the front is the right way to determine if the rear end is 3:42 or the 2:73. Is this correct?. I like to 4x4 in the summer and use the truck to plow snow in winter. It is a dual fuel which was added later to get better mileage. This is only a $1500 truck mind you and maybe I shouldn't be spending a lot of bucks on this. What are your thoughts on this please.

[75K30]

You will notice a note in my first reply, The note refered to the 4x4 requirement, and stated that any gear change will require the same gear in the front axle if 4 wheel drive.
At any rate, chevy used a solid front axle up to the 87 truck. I have to assume you are working on an 86 chevy K something. With a gear ratio such as a 2.73:1, I again have to assume a 1/2 ton.
To answer yur question, in order to verify the front differential ratio, one must remove the front cover, and either count the ring gear teeth and divide by the number of pinion gear teeth, or look on the ring gear for the number od teeth stamped right into the gear itself.
Making sure the front and rear diffs. have the same ratio is a must. Turning the two at the same rate while in 4wd is very important. Failure to do so will bind up the transfer case, and will damage it. chances are the case will break.
A good inspection of the front axle and differential is a very good idea.
FWIW: In my area a 10 bolt front axle with a decent ratio can be bought for around $150.00, and a rear 10 bolt with rear disc brakes are around $200.00 (US).
Many off roaders in my area sell these cheap after a larger axle upgrade.
You plow and use off road accasionally, what size tires are you running?
Was thinking even 3.42 might not be the best ratio either.
Will be interesting to see what ratio is in the front.

[436'd skylark]

its possible the the truck is a diesel with such a high ratio. if thats the case, you'll want the high gearing or else your top end will be severly limited

[75K30]

Originally Posted by 436'd skylark

its possible the the truck is a diesel with such a high ratio. if thats the case, you'll want the high gearing or else your top end will be severly limited

Most of the 6.2's from that era still ran 4.11's, and the military units had 4.56's, even with the diesel.
When I had a whimpy 6.2 it needed the help of the extra gear. Top speed remained the same.
When I ran a 6.2, i ran 5:13's with 40 inch tires, and could have used more gear. Newer SD diesels were available with 4.30's.
I am not a believer that a ratio changes top speed, unless the gear is so low that the engines max rpm is reached before a reasonable speed can be achieved, but that would take an axle ratio of way over 6.5:1 with a diesel, and little tires.
Would be willing to bet that a small 349 CI diesel from this era would not go any faster with 2.75:1 gears than the same vehicle with 4.56's.
Matter of fact, even the diesel engine would run better with the 4.56's.
The engine while making a bunch of torque at low rpm's just does not make enough to pull that kind of gear. Heak these only made torque figures around the 200 FT LB mark. Can't see it pulling that kind of gear.
Can't see even a common rail cummns pulling that kind of gear.

[75K30]

Best info I have says that a K-10 with the 2.73:1 ratio was only available from the 84-87 K series trucks with a 350 and a 4 speed manual.
A 3.08 ratio was available with a 305, and a diesel, but the diesel had an automatic only, and the gasser had to be a handshaker. A 350 with this ratio had to have the 700R4.
3.23:1, 3.42:1, 3.73:1, and 4.11:1 ratios were also options in these years, so anything is possible I guess.
Still say pop the cover, and the solution will be right there.

[ryiguy]

my c1500 1989 has a 350 with a 700r4 with 3.08 gears

[436'd skylark]

Originally Posted by 75K30

Most of the 6.2's from that era still ran 4.11's, and the military units had 4.56's, even with the diesel.
When I had a whimpy 6.2 it needed the help of the extra gear. Top speed remained the same.
When I ran a 6.2, i ran 5:13's with 40 inch tires, and could have used more gear. Newer SD diesels were available with 4.30's.
I am not a believer that a ratio changes top speed, unless the gear is so low that the engines max rpm is reached before a reasonable speed can be achieved, but that would take an axle ratio of way over 6.5:1 with a diesel, and little tires.
Would be willing to bet that a small 349 CI diesel from this era would not go any faster with 2.75:1 gears than the same vehicle with 4.56's.
Matter of fact, even the diesel engine would run better with the 4.56's.
The engine while making a bunch of torque at low rpm's just does not make enough to pull that kind of gear. Heak these only made torque figures around the 200 FT LB mark. Can't see it pulling that kind of gear.
Can't see even a common rail cummns pulling that kind of gear.

I forgot about this thread. was never sure what ratio diesels ran at. I figured it was pretty high since the old crappy 6.2s redline way low. 4.11s is way lower then I would have guessed, but what the **** do I know? even with the 4.11s the tires are so tall it really evens out some. see I'm more of a drag racer, specifically bracket racing. where selecting the ratio is a balance of trap speed vs. desired RPM to be consistent. The tires are 27", the rear ratio is 5.13s and we get about 135 at 7200 or so, IIRC. I was thinking more of the small tires acually, 4.11s, 27", and a redline of 3000 kind of limits the vehicle to about 60-65.