You may have four gallons but you can't use all four of those gallons. Most vehicles especially with larger tanks but it does vary depending on the shape, when they actually start sucking air and stall from lack of fuel still have close to a gallon or more in the tank.
Most of the Fords I have owned have had quite accurate fuel gauges. Now once a tank has been off in the fuel pump replaced all bets are off. It only takes a little bit heavy hand to bend the float arm a little bit and it will not be exactly like it was before. Or, if you're installing a fuel pump complete module like many people do you have a new float system which is not going to be the same as the old one.
I have run every vehicle I've ever owned with the distance to empty indicator down to 0 miles to empty and further.
It certainly depends on whether you're in stop-and-go traffic or if you're rolling on a Surface Road at 40 miles per hour versus driving on the highway at 70.
You would actually make it more miles right around 42 to 45 miles per hour on flat ground then you would on the highway because you're using more fuel to maintain that speed. But it's the Stop & Go in traffic and the idling that kills your miles per gallon versus Highway.
You can always go a few miles once you hit 0.
The older GM's on the other hand were highly inaccurate especially when they get down to the lower range. Most of the GM's I own don't even have distance to empty but I certainly wouldn't trust them.
You can be driving all day and come home and park AGM with two notches above the empty mark and then go back out 20 minutes later I started and it won't even go to empty but then in the first three fourths of a mile you drive it will go back up to three or four lines above empty. They have a crazy mind of Their Own.
