I have filled my 2013 F250's tank at least ten times now. There has never been a time that I have put more than 30 gallons in the tank. Even today I was on E and the lie-o-meter said 12 miles until E. New gas Super Duties are supposed to have a 35 gallon tank. Are there still 5 more gallons at E? I don't think a 30 gal diesel tank would work for a gasser would it? I really don't want to run it dry to find out if I really have a 35 gallon.
My 2011 F250 CCSB has a 35 gal tank. I never put more than 33 in it and there was still " 25-30 miles to go" according to the on board instrumentation. I generally go with the rule of thumb of 1/4 of a tank while towing or general highway driving, start looking. Around the town, 1/8 of a tank time to get some fuel.
My wife is ignore all indicators for low fuel. Wait until E. Then get fuel. My Explorer has a 22 gallon tank and it has never taken more than 20.5 gallons with the needle on E.