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?

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[foamypirate]

Welcome aboard!

As far as towing goes, I think most 1 ton squarebodies topped out (officially) around 10-12k or so, but it depended on gearing and equipment. There is a trailering special package that raised the GCW to 15,000, but I think it's fairly rare.

If it feels sluggish with the 8.1L and 4L80E, I'd definitely be looking at your rear gears to see what they are.

 

[Blake_496]

Welcome aboard!

As far as towing goes, I think most 1 ton squarebodies topped out (officially) around 10-12k or so, but it depended on gearing and equipment. There is a trailering special package that raised the GCW to 15,000, but I think it's fairly rare.

If it feels sluggish with the 8.1L and 4L80E, I'd definitely be looking at your rear gears to see what they are.

I'm running 3.73 gears and I think it's an open diff.

Currently running the stock tune on the engine but have tweaked the tranny tables to put it in 'tow/haul' mode all the time. The truck is only used for towing anyway. I know I can tweak the tune quite a bit.

so I wonder how I can tell if this was the HD version? It is a 'camper special'. If not, it sounds like I'm maxing out the towing already...

 

[yevgenievich]

4.56 rear gears might be needed in your case. 3.73 was a standard ratio for bb equipped c30.
Info states 4.56 required for 10k gvwr rating

 

[Blake_496]

ok so we think that the HD versions of the truck were the same as the regular one ton with the only difference being the gear ratio in the axle?

I have plenty of power and plenty of transmission. My concern is surrounding the fact that I may very well end up towing two rigs at the same time, which will put the trailer weight (with both rigs on it) at around 15k.

 

[foamypirate]

ok so we think that the HD versions of the truck were the same as the regular one ton with the only difference being the gear ratio in the axle?

I have plenty of power and plenty of transmission. My concern is surrounding the fact that I may very well end up towing two rigs at the same time, which will put the trailer weight (with both rigs on it) at around 15k.

Honestly, as long as you have a trailer brake controller and good trailer brakes, you'll probably be fine. Just need to drive carefully and take it nice and easy.

 

[kickdeez]

I don't know the answer to your specific question about towing capacity, but when you say you "could definitely tell it was back there", what exactly do you mean? Did you get the sense that you were under-powered? Or were you referring to the way the truck handled (suspension)?

If it seemed like there was a lack of power, then definitely consider your axle gearing, as previously suggested. I would think that you have plenty of displacement in that motor, and a strong enough transmission to tow what you are going to be towing. A good tune will also help wake that motor up.

However, if your reservations revolve around the way the truck handled with the trailer behind you, then I would look into upgrading the rear suspension with helper springs or air-bags. Loading up that double car with another rig in the front will definitely put a significant amount of tongue weight onto the rear of the truck.

I honestly don't think it will be a problem to tow what you are describing. You may need to make some adjustments to the suspension to get the handling you want, and/or some adjustments to the motor to get the power you want out of it. But I have seen lesser tow rigs pull equivalent loads.

 

[kickdeez]

Honestly, as long as you have a trailer brake controller and good trailer brakes, you'll probably be fine. Just need to drive carefully and take it nice and easy.

+1000 what he said ^ Good trailer brakes go a long way.

 

[Blake_496]

I don't know the answer to your specific question about towing capacity, but when you say you "could definitely tell it was back there", what exactly do you mean? Did you get the sense that you were under-powered? Or were you referring to the way the truck handled (suspension)?

If it seemed like there was a lack of power, then definitely consider your axle gearing, as previously suggested. I would think that you have plenty of displacement in that motor, and a strong enough transmission to tow what you are going to be towing. A good tune will also help wake that motor up.

However, if your reservations revolve around the way the truck handled with the trailer behind you, then I would look into upgrading the rear suspension with helper springs or air-bags. Loading up that double car with another rig in the front will definitely put a significant amount of tongue wait onto the rear of the truck.

I honestly don't think it will be a problem to tow what you are describing. You may need to make some adjustments to the suspension to get the handling you want, and/or some adjustments to the motor to get the power you want out of it. But I have seen lesser tow rigs pull equivalent loads.

well...

I bought this truck in August 2015 in bone stock condition (well to be honest it did have a 350 that someone had put in place of the 454). I almost immediately swapped in the big block/gearstar and I have not really driven it that much other than a 2000 mile trip to Arkansas to show my dad (he had the same truck back in the day). So I really don't know much about the truck other than it appears to be in AMAZING shape and literally 100% rust free.

The suspension and brakes work fine - even while towing. I think my comment about "could definitely tell it was back there" was mostly surrounding the fact that the truck is still kinda 'new' to me and it did seem to put a good strain on that 8.1 I have the HP tuner and moved all the tranny settings over from 'performance' to 'normal' which basically means the truck is in tow/haul mode all the time. It seems to hold the RPM's pretty high and would downshift from 4th to 3rd way more than usual. I guess I'm not used to that 8.1 having to strain AT ALL cause lemme tell you, that thing hauls @$$ when unloaded.

I built the truck specifically for towing and occasional DD work (I have a 5 mile commute).

I am planning to tow what you see above to Moab next week and that involves going up and over the continental divide at 10k+ feet and going thru some twisty mountain roads. I don't wanna get out there on the hwy and have some stupid fail like the frame, springs, etc because I am towing.

I wanna be safe.

 

[Blake_496]

oh and yes, all 4 trailer brakes work GREAT!!

 

[kickdeez]

I "bumper pull" about 10k with my 2500 suburban, and I wouldn't hesitate to run your setup any day of the week.

 

[kickdeez]

Also, I have noticed a huge difference in the way my motor works depending on how I load my trailer. When my trailer is really tongue heavy, my motor seems to have to work a little harder. But when I get all the weight over the trailer axles, she doesn't even break a sweat.

 

[yevgenievich]

if it is hunting between 4th and 3rd, leave it in 3rd.

 

[bucket]

If it's just used for towing, I would certainly swap a different ratio into the axle. Looks like you have around a 32" tire, 4.56 would work much better for what you use the truck for. If it ain't squatin', it brakes well, and it handles fine... don't worry about what the truck was rated for when new. It's a different animal now.

Is that the stock color?

 

[glockholiday]

My 2 concerns would be that stock radiator with the extra horsepower pulling in the mountains and the truck brakes.

I feel that I want my trucks brakes to be able to do an adequate job in the event my trailer brakes fail which I wouldn't trust in your position. Brakes are usually the limiting factory with towing capacities especially on these older trucks.

Just for reference, my 2002 F350 Dually 7.3 powerstroke is rated to tow 12,500 tongue pull and 12,500 5th wheel. I asked the dealer why it wasn't more for the 5th wheel rating like every other truck and he said it was based on brake performance. The year or 2 newer model of my truck stepped up a few thousand pounds towing capacity which was partially due to going up to a 17" wheel and larger brakes.

All that said, I would tow your pictured set up all day long.

But remember, your insurance may not cover any accident if you are over the weight that Chevy designated for that truck in 1975. Modifications don't matter to them.