New to me '97 K2500 'Burb with 2005 rear disc brakes/axle swap, Can you please help me identify what I bought?

[GotGoats]

Hey y'all- new here so thank you for bearing with me! I've got a gearing identification adventure to embark upon, and I need YOUR HELP (okay, maybe not THAT GUY)!

Just bought a 1997 K2500 GMC Suburban SL 6.5 Turbodiesel 4x4- 109k on the clock, red cloth/bench interior (front and back seats are MINT), FASS fuel system, new turbo, lots of new parts, originally in service as a fire chiefs truck. Has a bunch of holes from all the lights and signals but the body is straight, the frame is super clean, and not too rusty on the body (bottom of the doors and rear splash areas, but ultimately super clean)

The previous owner said the original owner swapped for what he remembers a disc brake/axle setup from he believes a 2005 but never changed the gearing and hasn't ever put it in 4x4 during his ownership

I am about to upload some pictures which will obviously make this identification much easier, but any clues where to look for identification of what the rear end might be from?

The transfer case engages fine and I haven't actually driven it in 4x4, I just will not drive it in 4x4 until I figure out what the gearing situation is and if I have to regear the rear.

I believe it's a stock 3.73 front, previous owner thinks the rears might be 4.10

The truck will be a tow pig/farm truck so I would also take opinions/thoughts on keeping the 3.73s versus matching the 4.10s- We tow a 30 foot tandem axle RV and a couple of tandem flatbeds and a bunch of small stuff. Nothing crazy crazy, just a bit bigger than my F150 is why I got it in the first place. It will sometimes go on the beach as well, for what its worth..

I'm a mechanic, so I will be doing the re-gearing if that's all it needs, any help and advice y'all can provide is so greatly appreciated. Glad to discuss offline as well.

Thank you all in advance, don't flame me too hard for being a Ford fanboy- I greatly respect and have been wanting to get into the GM game for some time. First GM for me is this one- How did I do?

Thanks,

Scott (GotGoats)

 

[Schurkey]

1997 K2500 GMC Suburban SL 6.5 Turbodiesel 4x4

The previous owner said the original owner swapped for what he remembers a disc brake/axle setup from he believes a 2005 but never changed the gearing and hasn't ever put it in 4x4 during his ownership

The 2005 axle is not a direct bolt-in. Either it's not a 2005 axle, or somebody has done cutting/welding to make it fit.

Which axle is this? Kinda guessing the original was a 10.5" full-float based on a turbo-Diesel 2500. My '97 K2500 7.4L has the 10.5 FF. Small-blocks would have gotten the 9.5" semi-float.

I'd be REALLY concerned that somebody DOWNgraded the braking system by stuffing a "1500" disc-brake axle in place of a proper "2500" axle/brake assembly. Folks think that disc brakes are magic; a crappy-small disc brake is somehow "better" than big drums. BE CAREFUL.

I am about to upload some pictures

Yes, please. In-focus, nicely cropped, resized as needed.

I believe it's a stock 3.73 front

What does the SPID decal in the glovebox show for gearing?
GT4 = 3.73
GU6 = 3.42
GT5 = 4.10

, previous owner thinks the rears might be 4.10

Pull the cover, look at the gears.
41 / 11 = 3.73

You must be registered for see images attach

The truck will be a tow pig/farm truck so I would also take opinions/thoughts on keeping the 3.73s versus matching the 4.10s-

The rear axle will be easier to change than the front...

We tow a 30 foot tandem axle RV and a couple of tandem flatbeds and a bunch of small stuff. Nothing crazy crazy, just a bit bigger than my F150 is why I got it in the first place. It will sometimes go on the beach as well, for what its worth.

...but it seems like you're going to be towing a lot, so the deeper gears may be a genuine improvement. I don't know about the torque of the 6.5TD especially as modified.

What transmission?

 

[Sean Buick 76]

That would have a 4L80e. Just verify the rpm at 70 mph using a gps and do the math to find the rear end ratio.

 

[Schurkey]

6.5TD never came with a manual trans?

 

[BNielsen]

6.5TD never came with a manual trans?

Not in Suburbans; not from the factory anyway.

 

[GoToGuy]

That would have a 4L80e. Just verify the rpm at 70 mph using a gps and do the math to find the rear end ratio.

That's great for speedometer. How do you correct for tire size, taller or shorter?

 

[GoToGuy]

What's your trailer weigh fully loaded? Your taking it to the beach parking lot or into the beach sand? Who in there right mind would stuff an axle assembly w/ gear ratio that doesn't match?!?

 

[Sean Buick 76]

That's great for speedometer. How do you correct for tire size, taller or shorter?

Use the bottom calculator. Use 0.75 for the transmission gear ratio which is 4th gear in a 80E

Engine RPM Calculator

 

[GoToGuy]

Cool, thanks.

 

[1998_K1500_Sub]

The transfer case engages fine and I haven't actually driven it in 4x4, I just will not drive it in 4x4 until I figure out what the gearing situation is and if I have to regear the rear.

Gears same / different? Do this... while stationary, put it in 4x4, put it in drive and give it a light throttle (maybe even no throttle). If it moves readily, the gears are the same. If it doesn't, they're different. It will be immediately obvious, and no harm done.

As may have been said earlier: Your RPO code should tell you what the factory installed in front. What's actually there may now be different, if you don't know the truck's complete history.

If you decide to re-gear for whatever reason, give consideration to your target driving speeds / engine RPM / transmission gearing and tire size, and use those objectives to pick a final drive (rear gear) ratio that meets those objectives.

In particular, it's nice to have gears that give you usable towing ability in OD and stout towing ability in D, all while meeting some speed and engine RPM objective.

For example, on my '98 L31 Suburban, with 265/65-18 tires and 4L60E, I changed to 3.73 gears (stock was 3.42) so that I would have:

~2200RPM in OD at 70MPH, which gives decent performance and some towing ability (ATVs on trailer)

~2900RPM in D at 65MPH, which is usable for towing as heavy a load as I care to pull (about 6500lbs, i.e., a vehicle on a low trailer); the L31 starts to die off much above 3500RPM anyway, generating more fan noise than torque.