Re: 390 improved fuel economy
I have a 58 TBird with a 352. Hoping to improve the gas mileage, I have
focused on the ignition system as most vehicles had poor ones in those days.
I started with replacing the coil on mine (which appeared to be the same one as used on Model A's - mustard colored top). I got an MSD coil and
ballast resistor. This meant substantially better spark.
Next I widened the plug gap from .034 stock to .042".
On the highway, this got me up to 17 mpg, from 14.
Currently I have just installed solid copper core Accel plug wires so voltage is not lost in the wire. I intend to widen the gap further to .050" and see what happens.
The additional draw of the MSD coil takes its toll on the points and I may change to a solid state Pertronix or something.
My goal is to get 20 mpg out of it - highway.
You could ask whether the carb can be leaned out any. Back in '66 there was no incentive to have a clean running motor so it may be on the rich side. If the carb has metering rod(s) then you could experiment with different ones for little money and not much effort.
john - MA