Intro and question about towing with a C35 one ton

[Blake_496]

Hey everyone! I am moderator of fsjnetwork.com where we talk about the old full sized jeeps. I built a '78 J10 Honcho a few years ago into a rock crawler and now I have built a '75 GMC C35 one ton 2wheel drive dually so I can tow the Honcho. My Dad actually had the exact same truck that he bought new in '77. Same color and everything. He had that truck on the farm and it rusted out a few years ago though.

I installed a 2005 8.1L L18 backed by a new gearstar 4L80E tranny into the C35. Swap notes include the following:

stock radiator
stock starter for a 454
stock headers for a 454
stock engine mounts
stock transmission crossmember that I moved to the rear position because the 4L80e is longer than the stock TH400

The truck is a 100% rust free Colorado truck and is in amazing condition. It rides and drives great. No issues with it at all.

My question is surrounding towing capacity. I've recently completed the truck and bought a 34' gooseneck trailer with the intentions of towing my rock crawler and maybe another rig if needed. The trailer weights 4600lbs and the J10 weights 6000lbs. I have a new B&W GN hitch (the one rated for 30K). I loaded up the jeep this weekend and took everything for a test drive. All went well, but I could definitely tell the trailer/truck was back there.

What is the max towing rating for these old C35's?

[slimneverdies]

Welcome to the site,

Your year truck GCW will depend on the rear gear that came stock. If it's a 3.73 it's 13,000 and if it's a 4.10 or 4.56 than its 15,000. You should also have trailering special GCW name plate right next to the window molding on the door if it's a trailering special model...

[daddyjeep]

That's a good looking combo there Blake. When I get my 85 Crew in good enough shape I will be dragging my crawler out to your neck of the woods.

[burban nate]

These old trucks aren't quite as heavy as newer ones, which causes it to feel like the trailer is pushing you around. For comparison sake after I bought my '90 V3500 crew cab I took it to the scale empty and it was right about 6500#. Shortly there after I took my dad's '97 k3500 crew cab to the same scale empty and it was just over 7500#. Both trucks are 454 automatic trucks. The '90 had the Dana 60/70 combo with the NP205 transfer case. The '98 had the ifs and a corporate 14 bolt rear end and the aluminum chain drive t-case.

You have the power with the 8100, I just think the lighter weight of the older trucks makes a huge difference.

Nate

[Burbnasty]

I have no answer to your question... However that is such a badazz combo, that whatever small towing disadvantages you may be experiencing you make up for and then some in style points!!

[rumblebox]

What accessory set did you use for the swap? I'm seriously considering an 8.1 swap with an allison or NV4500.

[KQQL IT]

Had a '86 1t with a roll back bed, and pulled a 24' open car trailer.
454 4 speed 4:10 gear doug nash od
Have had 2 '55 caddys on it. About 22k or so, Pulled it fine, stopping however not so much. Still 300k+ in it.

[Firebirdjones]

I believe Slim is spot on with the GVW's vs gear ratio. I have a 79 1 ton 454 with 3.73's and 13,000 GVW is what I recall.

I've pulled my 28 foot enclosed across country with it. Empty the trailer is almost 5,000 lbs., had a 4100 lbs. car in it, air compressor, generator, complete 5 foot tall tool box packed with tools etc... Every bit of 10,000 lbs. if not more and it did a fine job pulling it around.

[SilverMiner]

Originally Posted by slimneverdies

Welcome to the site,

Your year truck GCW will depend on the rear gear that came stock. If it's a 3.73 it's 13,000 and if it's a 4.10 or 4.56 than its 15,000. You should also have trailering special GCW name plate right next to the window molding on the door if it's a trailering special model...

According to the 1975 Chevrolet Truck Vehicle Information Kit from the GM Heritage Center website, there were three GVW ratings for the 1975 C30; 9000 (standard), 9600 (Camper Special RPO Z81/Z83), and 10000 (Dual Rear Wheel RPO R05). Tire sizes determined whether higher GAWR axle rating were given rather than gearing. To achieve the highest GVWR of 10K the RPO J55 HD Power Brakes had to be selected as well. Neither the Vehicle Information Kit or the sales brochures make any mention of GCWR, only GVWR.

[Firebirdjones]

Here you go...GCWR in pounds by axle ratio

[Firebirdjones]

Speaking 1 tons with a 454 it looks like
3.73's = 13,500 lbs.
4.10's = 15,000 lbs.
4.56's = 17,000 lbs.

[SilverMiner]

Originally Posted by Firebirdjones

Here you go...GCWR in pounds by axle ratio

Well sure, if you jump to the 1978 data. Given the minimal changes from 75 no doubt the info is applicable, but it's not the GM gospel for the OP's model year.

[Firebirdjones]

Isn't much GM says I'd take as gospel but it's a pretty safe bet those trucks went virtually unchanged from 73-79 so I wouldn't have a problem applying that chart to any of those years.

[v30crewcab]

I've had 20K behind mine on the gooseneck behind mine. tows and stops just fine, but I have 4wheel disc, and a little better than stock drivetrain! These are tough old trucks!

[Blake_496]

Hey guys, I'd like to post an update on my old square.

I built this thing 5 years ago with the intention of towing my rock crawling rigs to the trails. Then life changed and I ended up moving to an epic offgrid property 8500 feet in elevation.

I'm on a 6 mile dirt road that is very steep. The road is washboarded and bumpy.

I've been hauling water up here for over 4 years with the old dually (and a taco). Mostly been hauling 450 gallons at a time with the tank in the bed. That is 3300lbs and the truck handles it well. It's a 40 mile round trip run to Estes Park CO from my place. 14 miles of 45mph windy hwy and 6 miles of dirt road all uphill.

I ended up putting in 5.13 gears in the stock D70HD (on stock tires) and that really helped getting loads of water and materials up here.

Now here is my question based upon this rig '75 GMC C35 2wd with an 8.1 /gearstar 4L80E / 5.13 gears and detroit. Otherwise stock truck on stock springs.

My question is: Should I be worried about hauling 16klbs of gross vehicle weight with this old truck? It handles it prettty well, but man this 6 mile dirt road is rough and steep.

[kipps]

There's two things you need to worry about. What's legal to tow, and what's safe to tow.

Legally, you can't exceed the truck's GVWR. That's the weight on the two axles of the truck, including trailer tongue weight, but not including weight on the trailer axles. I'm almost certain you're good to go there.

Safe towing capacity is based largely on how the rig feels when driving it. If you're comfortable doing what you're doing, that's the most important thing.

[Dead Parrot]

As long as everything is in good shape and you don't bust the GVWR, the truck is doing what it was designed to do. But except for the new parts you swapped in, that truck is ~47 years old. You should keep in mind that on a steep rough road at altitude, things can go from great to crap really fast. Going down that bumpy hill with 16k and finding out you forgot to connect the trailer brakes.... (or a mouse chewed on some wires to the same effect).

[Keith Seymore]

Originally Posted by kipps

There's two things you need to worry about. What's legal to tow, and what's safe to tow.

Legally, you can't exceed the truck's GVWR. That's the weight on the two axles of the truck, including trailer tongue weight, but not including weight on the trailer axles. I'm almost certain you're good to go there.

Safe towing capacity is based largely on how the rig feels when driving it. If you're comfortable doing what you're doing, that's the most important thing.

Originally Posted by Dead Parrot

As long as everything is in good shape and you don't bust the GVWR, the truck is doing what it was designed to do.

While I appreciate the sentiment(s), please be reminded we are talking about GCW (Gross Combined Weight), not GVW (Gross Vehicle Weight). The GVW is highly visible, whether on the SPID label or on the placard, but the GCW is a bit more nebulous since it varies by rear axle ratio.

Originally Posted by Blake_496

I ended up putting in 5.13 gears in the stock D70HD (on stock tires) and that really helped getting loads of water and materials up here.

Now here is my question based upon this rig '75 GMC C35 2wd with an 8.1 /gearstar 4L80E / 5.13 gears and detroit. Otherwise stock truck on stock springs.

On one hand, you are in uncharted territory since the factory did not offer the 5.13 rear gear. On the other hand, you are inside the 17,000 lb chassis limit (the highest rating shown on the chart) so you should be ok.

Having been the one that established the trailering rating for some of these vehicles I can say the limiting factor was the ability to launch the vehicle at GCW up the 16% grade. There were a number of combinations that would have been rated higher if it were not for this particular requirement.

K

[Blake_496]

I don't tow anything DOWN the mountain. It's always UP the mountain.

The truck drives and handles and brakes great when loaded. I only have the Gooseneck. No bumper pull.

I guess I worry about the integrity of the leaf springs and the frame. They all look good, but ~46 years is a long time.

[kipps]

Originally Posted by Keith Seymore

While I appreciate the sentiment(s), please be reminded we are talking about GCW (Gross Combined Weight), not GVW (Gross Vehicle Weight). The GVW is highly visible, whether on the SPID label or on the placard, but the GCW is a bit more nebulous since it varies by rear axle ratio.

Is gross combined weight rating even legally enforceable if it's not written on the vehicle anywhere? Are DOT officers expected to scrounge up 40 year old sales brochures to find the critical numbers?

As far as I can tell, the only number that matters to law enforcement is the GVWR. I welcome correction though.

Edit: Was the GCWR a legally binding thing back in the squarebody days? It looks a lot more like a factory "recommendation" that might affect warranty claims at best. After all, there's nothing inherently unsafe about towing a heavy load with a 3.42 axle ratio. DOT doesn't care how heavy towing affects your drivetrain. They just want to know your chassis, tires, and brakes are big enough.

[Keith Seymore]

Originally Posted by kipps

Is gross combined weight rating even legally enforceable if it's not written on the vehicle anywhere? Are DOT officers expected to scrounge up 40 year old sales brochures to find the critical numbers?

That is an excellent question, and I do not know the answer. It is hard enough for us enthusiasts to find, so I wouldn't expect law enforcement officers to have this arcane knowledge.

Originally Posted by kipps

As far as I can tell, the only number that matters to law enforcement is the GVWR. I welcome correction though.

Correct.

My point was that GVW and GCW are two different animals.

Originally Posted by kipps

After all, there's nothing inherently unsafe about towing a heavy load with a 3.42 axle ratio. DOT doesn't care how heavy towing affects your drivetrain. They just want to know your chassis, tires, and brakes are big enough.

That was my point. The chassis, tires and brakes don't change with the rear axle ratio. The only thing the axle ratio affects is launchability.

K

[Keith Seymore]

Originally Posted by Blake_496

I don't tow anything DOWN the mountain. It's always UP the mountain.

My trips feel like they are uphill both ways as well.

K

[kipps]

Originally Posted by Keith Seymore

The only thing the axle ratio affects is launchability.

To some degree, it also affects the amount of stress on the engine and transmission. A 3.42 will run a higher risk of clutch damage than the 4.10 ratio will. Lugging an engine is not good for its longevity either.

[Blake_496]

Originally Posted by kipps

Lugging an engine is not good for its longevity either.

I've never heard this. Can you elaborate?

My old dually has 5.13 gears and stock sized tires, so lugging the engine isn't something that happens much

[kipps]

Originally Posted by Blake_496

I've never heard this. Can you elaborate?

My old dually has 5.13 gears and stock sized tires, so lugging the engine isn't something that happens much

Every stationary International Harvester engine I've seen carries a tag saying "Do not overload" in big lettering. Lugging an engine increases the chances of bad vibrations in the drivetrain, which can chew up transmissions. The nv4500 transmissions had problems with this. Any slightly loose tolerances in the engine internals can also be exacerbated by the low speed lugging.

On a typical tractor diesel that's pulling maximum hp, it's at its most efficient when it's running at peak torque rpm. This is why most engine/generator setups that use similar engines are 1800 rpm instead of 900 or 3600. Obviously, the most efficient speeds are different at lower hp output levels.

It feels good to lug a big engine, but it's not the best for all the parts involved, nor for fuel consumption.