The F150 was available with two tank sizes in 1999, 24.5 gallons and 30 gallons, plus a second tank could be installed as an option. So, unless you know what size tank it has, we cannot tell you. And also it depends on other factors. In what kind of terrain? Mountainous, Hilly or flat? In what kind of conditions - open freeway, congested freeway, urban streets, country roads? Is the vehicle in perfect condition so that it gets its best mileage? Without any of that information it's really impossible to say. When it was brand new, a base model F150 with smaller tank in average driving on reasonably flat roads 60% highway, 40% city could get about 18 mpg, so about 430 miles. Now its so old and probably not in good tune, I wouldn't give it better than 14 mpg and 330 miles. Worse in traffic or city driving, worse in mountains, and way worse if you drive with a lead foot.