I have replaced all my transmission fluid with Mobil 1 ATF and I have increased my gas mileage by 1.5-2 mpg. Used a total of 14 quarts with some spillage.
Note that the FSM says that the drain and fill capacity is 2.1 quarts. I found mine was a lot more than that. More like 4 quarts. Be careful.
I currently drive 75% highway and 25% city and get around 19-20 mpg.
Another change is long ago, I removed my trailor hitch (40 lbs or so...).