I viewed a popular mechanics article from '82 stating an mpg of 15-ish.
I've read other posts stating that they had an overdrive activated by a foot dimmer switch. Not sure if that was a custom job or not but that could explain a large drop in mpg. I believe it had a TH400 tranny. The poster i read stated that the OD was on the tail end of the tranny (probably custom but worth a look). Others have stated 14 cruising at 70mph. I found a local for sale and they stated using 75% of the tank for a 220 mile highway ride. He didn't know that the tank was 40 gallons. That would make 22mpg. Even if he was stretching the truth and was under 75%, that would still come in over 15mpg fairly easily.