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Iam looking at buying the holley 390 cfm carburetor for my 3100 1st series pickup. I have a 600 cfm on there now and I think it is a little too much carb. I would think the mpg would be better. Any ideas on the matter? I do have a little RV cam in now .

Here are two formulas that return in almost the same results.
1. "(CID/2) * (RPM/1728) = CFM"
2. "CID * 2 * .8 = CFM"

* is multiply

Got them from a discussion in another forum. These made the most sense to me.
Plug in your Cubes and your close.

Almost the same results at what RPM?

If I did it right using #2 235x2x.8=376cfm Sounds like 390 400 Holley would be in the ball park. Now does anyone have that on there truck and how's the MPG . Thanks

RPM is a variable to enter, sorry, missed that. It could be entered as the target RPM for known performance of a cam per say.

The two equations give about the same result at about 5,500 RPM.

That is a fairly high RPM for a 235, I think?

Ya, unless built to race.
Lots of other variables to consider as well. I would used it as a baseline figure to round up or down depending on what my goal and other factors are (headers, blower, cam, altitude, etc) vs. what is available.

I know there are many more formulas and opinions and CFM is not the end all. Now you get to tune it.