Help locating GVWR, etc for 79 K20

[blink32]

So I've been out searching for some information for a 79 K20 and a 76 K10. The 76 I shouldn't have any problems with as I have the actual vehicle and the Owners Manual at home. But the 79 is giving me problems. I am looking for as much as possible of the information below so I can appropriately judge how much pay load I can carry. I plan on using the 76 to occasionally carry a Truck Camper but I will be utilizing the axles from the K20 (8lug D44 & 14bff) for the additional strength. I will also be upgrading the springs and using overloads or air bags as helpers.

Does anyone know where I can find the information below? As I said, I no longer have the actual 79, just some parts, and no owners manual/door tag. I want to stay as close to GVWR and GCWR as possible when sizing a TC.

Thanks in advance.


GVW: Gross Vehicle Weight - the actual weight of a vehicle.

GVWR: Gross Vehicle Weight Rating: The maximum allowed GVW for a vehicle.

GCW: Gross Combined Weight: The total weight of everything - tow vehicle, trailer, etc.

GCWR: The maximum allowed GCW.

GAW (front or rear): The Gross Axle Weight - the amount of weight on that particular axle.

GAWR (front or rear): The Gross Axle Weight Rating: The maximum allowed GAW.

[blink32]

ttt

[Captkaos]

GVW isn't the Actual weight of the vehicle, it is the GROSS weight of the vehicle loaded at X point in time, the maximum Weight is designated GVWR.
Curb weight is the actual weight of the vehicle.

Same goes for all the others. GCW is how much you actually have combined at X time, along with GAW.

GVWR for a K20 depends on what options it came with. The range is 6800-8400

No idea on GCWR

GAWR for Front: std - 1850, optional = 2250
GAWR for Rear: std - 2100, optional = 2800

[blink32]

How could the GAWR be less than the GVWR? If I'm reading your example choosing the max GAWR's it add's up to 5050. But the lowest GVWR is 6800.

I copied all those acronyms and definitions off a site. Thanks for the clarification.

[Captkaos]

Well, because GAWR is the maximum amount that the particular Axle can carry and is measured at the tires, and it includes the weight of the vehicle that is over the tires.

If you put 2200 lbs over the rear axle and your trucks curb weight is 4000, the total GVW is 6200, which is 600lbs less than maximum.

If you had a over the cab camper and it had 1000lbs over the front axle and 2000lbs over the rear axle and the truck weighs 4000, you GVW is 7000, which is 200lbs OVER GVWR.

If you are adding the springs and axles to you 1/2T 76, none of this is valid since the rating was based on a LWB 3/4T truck.

[KIILew]

Blink:

This doesn't really address the discussion your having with Captkaos, but I thought I would present some additional info here...

For 1976, the K10 pickup was offered with just one GVWR: 6,200 lbs. The 1979 K20 you were considering, however, (or for that matter, a 1976 K20) could have had any one of three GVWR’s: 6,800; 7,500; or 8,400 lbs. Here are some figures you might consider, compiled from the 1976 and 1979 Chevy Truck Data Books (and some 1976 and 1979 brochures, as well). To keep things simple, I have determined the curb weights for only the basic vehicles described (and accounting for any necessary hardware to achieve the noted GVWR’s); additional optional and aftermarket equipment would increase these weights, and therefore reduce the payloads relative to what I have shown below. Still, the figures below give some idea of the differing capabilities of these GVWR packages.


Specifications for 1976 K10 @ 6,200 lb GVW w/350 4bbl V8/SM465 and Tubeless Tires --

Frame Section Modulus: 3.92

Front Spring Capacity: 1,850 lbs each
Front Axle Capacity: 3,600 lbs. each
Front Tire Capacity: 1,760 lbs. each
Front Stabilizer Bar Diameter: 1.25”
Front Shock Absorber Diameter: 1”
Front Brake Rotor Diameter: 11.86”
Front GAWR: 3,520 lbs.

Rear Spring Capacity: 2,000 lbs. each
Rear Axle Capacity: 3,750 lbs. each
Rear Tire Capacity: 2,800 lbs. each
Rear Shock Absorber Diameter: 1”
Rear Brake Drum Dimensions: 11.15” x 2.75”
Rear GAWR: 3,520 lbs.

GVWR: 6,200 lbs.
Curb Weight of vehicle: 4,564 lbs.
Difference = Payload: 1,636 lbs.

GCWR (w/3.73 axle ratio): 9,500 lbs.
GCWR (w/4.10 axle ratio): 10,500 lbs.
GCWR (w/4.56 axle ratio): 11,500 lbs.




Specifications for 1979 K20 @ 6,800 lb GVW w/350 4bbl V8/SM465 and Tubeless Tires --

Frame Section Modulus: 3.92

Front Spring Capacity: 1,850 lbs each
Front Axle Capacity: 3,800 lbs. each
Front Tire Capacity: 1,990 lbs. each
Front Stabilizer Bar Diameter: 1.25”
Front Shock Absorber Diameter: 25mm
Front Brake Rotor Diameter: 12.5”
Front GAWR: 3,700 lbs.

Rear Spring Capacity: 2,100 lbs. each
Rear Axle Capacity: 5,700 lbs. each
Rear Tire Capacity: 1,990 lbs. each
Rear Shock Absorber Diameter: 25mm
Rear Brake Drum Dimensions: 11.15” x 2.75”
Rear GAWR: 3,980 lbs.

GVWR: 6,800 lbs.
Curb Weight of vehicle: 4,738 lbs.
Difference = Payload: 2,062 lbs.




Specifications for 1979 K20 @ 7,500 lb GVW w/350 4bbl V8/SM465 and Tubeless Tires --

Same as those given for 6,800 lb GVW except for the following:

Rear Spring Capacity: 2,800 lbs. each
Rear Tire Capacity: 2,350 lbs. each
Rear Brake Drum Dimensions: 13.0” x 2.5”
Rear GAWR: 4,700 lbs.

GVWR: 7,500 lbs.
Curb Weight of vehicle: 4,816 lbs.
Difference = Payload: 2,684 lbs.




Specifications for 1979 K20 @ 8,400 lb GVW w/350 4bbl V8/SM465 and Tubeless Tires --

Same as those given for 7,500 lb GVW except for the following:

Front Tire Capacity: 2,780 lbs. each
Front GAWR: Remains at 3,700 lbs.

Rear Tire Capacity: 2,780 lbs. each
Rear GAWR: 5,560 lbs.

GVWR: 8,400 lbs.
Curb Weight of vehicle: 4,869 lbs.
Difference = Payload: 3,531 lbs.




There is some variability in terms of GAWR; individual pickups, as reflected by the decals affixed to the driver’s door pillar, might have different GAWR’s for a given GVWR than those shown above. This is because GM offered a number of different tire upgrades within each GVW package. For instance, your current K10 might indicate a front GAWR of 3,520 as shown above, or it might have been originally equipped with heavier duty tires (or the optional heavy duty front springs described below) that would warrant a higher rating. Regardless, your K10 will indicate a GVWR of 6,200 lbs.

It is important to understand that higher gross axle weight ratings allow a greater portion of the total load weight to be positioned over the front or back axle, as the case may be. However, the total weight of the load and vehicle should never exceed the gross vehicle weight rating of the vehicle, even though the sum of the stated front and rear GAWR's assigned to the vehicle may exceed the stated GVWR.

Not surprisingly, Chevrolet recommended the heaviest K20 model exclusively, with the 8,400 lb GVWR, for carrying large cab-over campers. Since you have only some parts left over from the original truck, you need to determine if your K20 had the heavier duty rear brakes and 2,800 lb rear springs that characterized this maximum GVWR. To do this, you will need to measure your brake drums and count the number of leaves in your rear spring packs. The 2,800 lb spring pack you need will have 7 leaves, while the lighter unit will only have 6 leaves. If you combine these components with the appropriate tire capacities (2,780 lbs +), theoretically you could achieve the 8,400 lb rating.

Chevrolet also advised prospective camper buyers to purchase the optional heavy duty front springs for K20’s expected to carry campers. These springs were rated at 2,250 lbs each and can be identified as they were comprised of 3 leaves vs. the standard 2 – leaf pack that was rated at 1,850 lbs. In an 8,400 lb GVWR K20, these optional springs would raise the front GAWR to 4,500 lbs. However, the total GVWR would remain at 8,400.

With respect to trailering capabilities, the GCWR’s shown within the 1976 K10 info presented above should be relevant. However, if you intend to tow a trailer or fifth wheel extensively there are some other issues to consider. But we will save that for later!

I hope some of this information is useful!

Ken Lewis

[blink32]

Originally Posted by KIILew

Blink:

This doesn't really address the discussion your having with Captkaos, but I thought I would present some additional info here...

With respect to trailering capabilities, the GCWR’s shown within the 1976 K10 info presented above should be relevant. However, if you intend to tow a trailer or fifth wheel extensively there are some other issues to consider. But we will save that for later!

I hope some of this information is useful!

Ken Lewis

Lots and Lots of useful numbers. I appreciate it a lot Ken. And with this setup, if I ever did happen to tow, it would be very very light towing while camper was on. 4x8 or smaller trailer with sub-1000 lbs. But I don't anticipate even doing that.

[blink32]

Originally Posted by Captkaos

Well, because GAWR is the maximum amount that the particular Axle can carry and is measured at the tires, and it includes the weight of the vehicle that is over the tires.

If you are adding the springs and axles to you 1/2T 76, none of this is valid since the rating was based on a LWB 3/4T truck.

So the number you listed for GAWR are in addition to the curb weight? Meaning I could potentially carry up to 2800 additional lbs just on the rear axle if the vehicle came prepared in that trim? I'm just trying to make sure I understand this correctly.

And why wouldn't it be valid if I was adding the springs and axles to the 1/2 ton LWB? That is, in-fact, what I am going to do so I would like to know. I understand there could/will be differences and these numbers will just be guidelines but...

And for anyones info, I plan on choosing the lightest camper that will fit my needs, and if it ends up being borderline and close to limits, I will eliminate options in favor of a lighter load for a greater margin of safety. Of course taking into account the weight of interior items and belongings that aren't included in wet/dry weights of the campers. I'm not looking to try to fit an 11' Alpinelite with a dry bath weighing in at 3500lbs in the back of my pickup.

[KIILew]

Blink:

It appears to me that some of the numbers Chris gave you for GAWR’s are rather the standard and available spring capacities for K20’s. The sum of the front and rear GAWR’s, as you originally seemed to understand it, do add up to a number that will meet or exceed the GVWR in all cases. Although it may seem like a chore, you can see this if you reexamine my earlier post.

Having said that, and after reading your last post about trimming down your camper setup to accommodate your existing ¾ ton truck hardware, it seems all you really need to do is determine which springs, brakes and tires your K20 originally had. Once you nail this info down, and then sift through the data book miasma I’ve listed above, you can establish which of the three K20 GVW’s would apply to your old ‘79. Given that long-wheel-base K10’s and K20’s shared a common frame (with minor differences between model years rather than between series during the ’73 to ’80 time frame), it would seem that you could bring your K10 up to your previous K20’s capabilities with respect to load carrying by swapping components. Just make sure that the conversion is complete and that no stamina - compromising ½ ton pieces are left on the vehicle!

With respect to computing GAWR’s for these trucks, it may help to understand how to compute it. For the front axle you take the spring capacity for a single wheel and compare it with the tire capacity for that wheel, then take the lighter of those two capacities (i.e. the “weakest link”) and multiply it by 2 (to represent both wheels on the axle). You do the same to determine the GAWR for the rear axle, too.

So, if you have a K20 front axle with the standard 2 – leaf 1,850 capacity front springs and 2,780 lb capacity tires, you would calculate the front GAWR as 1,850 x 2 = 3,700 lbs.

If your K20 front axle was originally equipped with the optional 3 – leaf 2,250 lb capacity front springs and 2,780 lb capacity tires, your front GAWR would be 2,250 x 2 = 4,500 lbs.

If your original front axle setup combined the optional 2,250 lb capacity front springs with the 1,990 lb capacity tires found on the 6,800 lb GVWR K20, your GAWR would be 1,990 x 2 = 3,980.

If your rear axle was originally equipped with the 2,800 lb capacity springs found on the 7,500 lb and 8,400 lb GVW packages, and you combined this with 2,780 lb capacity tires, your rear GAWR would pencil out to 2,780 x 2 = 5,560 lbs.

…and so on…

Of course, even heavier duty suspension components and tires are available from the aftermarket. One just has to remember that such items won’t raise the GAWR beyond the capacity of the axle housing itself.

Good luck in sorting it all out!

Ken Lewis

[blink32]

Perfect. I'll use all that info to my advantage. The stock 79 springs were trashed with broken leaves, rusted through brackets, etc, so they were ditched. But now I have a much better grasp on how to calculate what I can carry vs what I weigh. I'll have to find a local CAT scale and get weighed. Thanks Ken.

Tony

[KIILew]

Tony:

Glad that some of the info is helpful.

I should add one little clarification/correction to my first post above that might be relevant to you if you use your K10's original front springs (or go spring shopping). Chances are your '76's front springs are the standard 2-leaf 1,850 pound variety. But, if they are the optional heavy duty 3-leaf pieces, and they are original to the truck, then, they will carry a capacity of 1,900 lbs (as opposed to the '79-issue 2,250 lb units that I described earlier). This is because GM upgraded the heavy duty front spring option to the 2,250 lb capacity for the '77 and later model years. The other spring capacities I listed are consistent for both '76 and '79 model years.

Incidentally, I just looked at the pictures you have of your '76. Very nice truck!

Ken Lewis

[blink32]

I do have the stock 2 leaf'ers. I've pretty much decided that I must go spring shopping and be very careful about it. I also will be getting some E load range tires and probably upgraded wheels to handle more weight. I'm shooting to stay sub-2000 lbs wet & fully stocked. It's going to be tough but as I said, if I have to give up something I will. I appreciate your help tons.

Thank you for the compliements. Its essentially stock. I drove it out from California about 3 years ago and spent a majority of the time making it safe. Grandfather bought it new in 75 and after he passed Grandma didn't want to sell it to a neighborhood person and see it driving by.

tony

[86k10]

I've always found it interesting how they rated these trucks. My 74 C20 has a gvwr of 6400 and a friends 75 K10 has a rating of 6500lbs but my C20 has more springs and bigger brakes. I don't understand that. I wouldn't think you would have any trouble carrying a truck camper even as a half ton, or is it heavier than I'm thinking.

[KIILew]

Tony, I can understand your grandmother’s thinking; my own family made similar efforts once to avoid seeing one of our former cars cruising around town. Nevertheless, it’s great that your truck managed to stay in the family!


’86k10…

Check your friend’s ’75 K10, again…if it was originally was a V8-powered pickup, I bet you will find that the GVWR was 6,400 lbs, like your C20.

Nevertheless, your point stands!

I also find the overlap in capabilities between ½-, ¾-, and 1-tons to be interesting. During the 1975-1980 model years, for example, GM offered a Big 10/Heavy Half with a 6,200 lb GVWR alongside the lightest-weight C20 which carried the 6,400 lb rating. Once, in an earlier post on this message board comparing differences between series, I noted that these two vehicles – when optioned with identical engines and transmissions – had nearly identical payload capabilities. In fact, because of the additional curb weight of the C20, the Big 10 actually offered – according to the GM data book – almost 90 lbs more payload carrying capacity than the heavier duty ¾ ton!

I used to wonder why GM offered this seeming redundancy. After all, why would someone buy the ¾-ton when the less expensive heavy-duty ½-ton would actually carry a comparable payload? A comparison of the specs reveals that these two pickups had identical front and rear springs, shocks, and even similarly-sized rear brakes.

Ultimately, I suspect that a buyer who would only occasionally load his truck to full capacity would find the ½-ton to be adequate for his purposes. Furthermore, GM’s marketing gurus probably reasoned that this likely “casual” buyer would balk at the notion of buying a ¾-ton. I mean, really, aren’t those just for campers and construction guys…???

I am sure that to other truck shoppers, however, the light-GVWR C20 made sense. Their insight told them that given a comparable load, the ¾-ton would handle that load with less wear and tear over the long haul. This superior stamina derived from the heavier-duty tires, wheels, front brakes, rear axle, and frame found on the 20-series truck. And, contrary to what you might read in some postings, on the 2-wheel-drive regular-cab trucks, the frames were different between series. A close examination of the data books reveals that while the C20 frame rails shared height and width dimensions with the C10 and Big 10 frames, the material thickness of the rails was greater than that of the C10’s/Big 10’s and actually matched that of the C30! (The C30 frame gained its strength advantage in having taller and wider rails than those found in the C20, although the material thickness was the same.) Still, regardless of load, the 6,400 lb-rated C20, with its comparable springing, sacrificed little in terms of ride quality to the Big 10.

Similar considerations were faced by buyers comparison shopping high-GVWR-20’s to low-GVWR-30. The comparisons were even more involved, since C20’s offered GVWR packages from 6,400-8,200 lbs, and C30’s ranged from 6,600-10,000 lbs (during the 1973-1978 time frame).

Now, back to your ’74 C20/’75 K10 comparison…your truck might have been optioned with either one of two heavy-duty rear spring options (2,600 or 2,850 lb capacity) instead of the standard 2,000 lb capacity rear springs that were found on most 6,400 lb GVWR C20’s – and all ’75 K10’s. That might explain why your springs appear heavier than those on your friend’s truck. As GM’s literature shows, heavier duty springs alone would not raise your C20’s GVWR beyond the 6,400 lb rating. They would only have raised your GVWR to 7,500 or 8,200 lbs if teamed with optional high capacity tires and rear brakes.

Later edit – another thought occurs to me, although I presently don’t have access to my data or parts books to confirm it. I believe that the K10 used a 52” rear leaf spring (2,000 lb capacity), while the C20 (and C10/Big 10) 2,000 capacity spring was a 56” unit. Since the shorter 4x4 spring was less flexible it used fewer leaves than the longer 4x2 spring (6 leaves vs. 8?). That could also account for the appearance difference.

Just some possible explanations!

Ken Lewis

[blink32]

Originally Posted by KIILew

Later edit – another thought occurs to me, although I presently don’t have access to my data or parts books to confirm it. I believe that the K10 used a 52” rear leaf spring (2,000 lb capacity), while the C20 (and C10/Big 10) 2,000 capacity spring was a 56” unit. Since the shorter 4x4 spring was less flexible it used fewer leaves than the longer 4x2 spring (6 leaves vs. 8?). That could also account for the appearance difference.
Just some possible explanations!
Ken Lewis

I believe this is true Ken. I had previously purchased a set of 4" lift springs for the K10 and the common question was 52 or 56 " springs? I have the 52's and was never able to find 56" on any of my model years that I saw in the Jyards.
Tony