Thread: Weird thing done to 305, what cam is it?

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First things first, welcome to the forum! I'm afraid your post is going to bring up more questions than answers at first. First, the 305 is not going to be a performer for you in power or efficiency, especially in something as heavy as your El Camino. You're right that the cam is not stock, and that could easily be a part of your problem if the previous owner bought the cam for the idle instead of for improved power & efficiency. The rule is that you select your cam to optimize your engine's power & torque, based on how you plan to use it, and the idle will take care of itself. Unfortunately there's no way to know the specifics of your cam without some pretty major disassembly of your engine. Let's go through some of the basics to see if anything jumps out:
1) Is that 12.5 mpg around town, on the open road, or combination? If it's around town or even combination town & some runs into Overland Park, Lenexa, NKC, etc it's really not that bad, especially if you don't have overdrive. If you still think it's too low, then....
2) What carburetor are you running, to see if the CFM of the carb is in the right range for your engine?
3) Automatic transmission, and do you know what it is by number?
4) How's your compression on each cylinder?
5) What's your ignition system? Distributor type? Any aftermarket "box"?
6) How do your plugs look after a good highway run?

Other guys may come up with other questions, too, but if you answer back with these it will get some dialog started. You might want to start saving your money for a nice 350 crate engine, or even a built out 383 crate from GM Performance Parts with a nice over drive auto.... Again, welcome to CHR.