The YF2100s is popular around here because it has a manual choke. That carburetor was designed in 1953 (or 1952 for the 1953 235). The 235 changed notably in 1959. If economy is what you want, the engine you have is a very good candidate, but just changing the carburetor isn't the whole answer. You can read about what GM did on their Heritage Pages. Essentially they went with hydraulic lifters and a camshaft that produced greater torque at a lower rpm. Doing this achieved an impressively long flat net torque line. Take a look at page 59 of this report:
https://www.gm.com/content/dam/comp...hevrolet-trucks/1959-Chevrolet-Truck.pdfThe net torque reaches almost maximum at 1600 rpm and stays flat almost until the net torque line and net horsepower lines intersect (around 2600 rpm). At 2000 rpm your 235 is producing as much torque as it will produce. It is not common to find a torque line like that...almost 1000 rpm at more/less max net torque is neat. From the standpoint of economy, this has a message. It means you can lower the overall gear ratio in your truck and zip along the highway using lower rpms. That change more than any carburetor you will ever find on the face of the earth will positively impact your driving economy. In my case, at 60mph my engine is rotating at around 2000 rpm. Maximum torque and good efficiency. With the stock rear end/transmission my engine would be rotating at around 3000 rpm at the same speed. 3000 rpm is 50% faster than 2000 rpm. Not max efficiency in any way. In fact at 3000 rpm I am well into what I call the "inefficient frontier."
The YF3200s was the last carburetor Carter engineered for the 235. Either that one or the 2100s will be fine for your 235...just depends on whether you're set on having a manual choke or whether you are happy with an automatic choke. Either of those two carburetors will be about the same regarding efficiency, although the 2100s was designed to run richer at lower speeds (in town driving), so your in town mileage may be a bit less. Good luck. We've been talking a lot about gear ratios here lately. Might be some interesting reading.
