That's exactly it. You have to monitor the fuel economy of your car on a regular basis. Most of the time poor economy is the result of a poorly tuned engine. This can be the result of lack of proper maintenance, or poor techniques.
Also, as in the case of cars built in the 70's and 80's before the advanced engine management systems we have today, many were somewhat detuned from their potential in the interest of low emissions. One of the more common approaches was retarded ignition. Others such as cam timing and profile, along with EGR systems stole away the efficiency of the engine. There is not much you can do about camshaft timing and profile unless you replace the cam, but spending some time with the car and working out your own tuning techniques can yield both better performance and fuel economy. Remember, that was how successful racers did it in the old days. Much was astute tuning.
But, outside of tuning and maintenance, other factors can contribute to both poor performance and fuel economy. Engines with excessive wear resulting in lower compression in any or all cylinders, and a worn timing chain could be the bigger factors. These are not so easily fixed, though.
Throughout the 50's, Cadillacs were celebrated for their fuel economy that rivaled or bettered lesser, lighter cars. This carried on pretty much through the 60's and into the 70's. I'm not saying that you will get 6-cylinder economy from a 390 or 429, but there is the potential for decent fuel economy by paying attention to the details.
I have proven that for myself, and the numbers I have posted are not whimsical numbers, but real numbers. In the case of the '75, I worked out the calculations three times before I convinced myself that the number was indeed correct.