A quart of old style ATF added into my 36 gallon fuel tank would get me about 1.5mpg more than running without it, and ATF has a lot of detergents so it helps to keep the injectors clean.
However, when oil prices spiked about 15 years ago so did the cost of a quart of ATF, actually ATF spike considerably, so it wasn't worth it for me to use it regularly. I haven't checked prices lately, mainly because oil prices are so high. But working on my various vehicles the past few years I noticed that most newer varieties of ATF have added friction enhancers so I am not sure how that would be good in the fuel system and in the engine. If oil prices drop next year (hopefully when we drill baby drill) I will check pricing on the old formula ATF again.
My truck takes 3 gallons of oil, plus the other vehicles I work on all use 6 or 7 quarts each. I filter my old oil and dump a few gallons into my mostly full diesel tank and let the engine "recycle" it for me.