Help locating GVWR, etc for 79 K20

[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