As previously mentioned, it's pointless to compare miles per tank when there are so many different varialbes, not to mention the different sized gas tanks in the Sport Trac.
You need to take accurate readings of gas consumed and miles driven. You cannot do that accurately on one single tank of gas unless you fill it to the brim where you can see it in the filler pipe, drive all you can and fill it back up to the exact same level as you started. Now you know exactly how much gas you used. divide the miles driven by the gas consumed and you have your average MPG for that tank of gas.
Any other way of measuring fuel mileage that relies on the gas gauge is just a wild guess.
If you had a trip computer that gave an instantaneous MPG readout you would see that your vehicle will produce MPG readings from Sero (0) to probably as high as 35-40 MPG, depending upon if you are idling (=0 MPG) to coasting down hill with no accessories on (=35-40 MPG). If you can find a destination that is down hill both ways, you could get over 30 MPG in your Sport Trac under ideal conditions.
If you use E85 or gasoline that contains alcohol, you will get a little less mileage than you would if you used pure gasoline. Much of the lower 87 octane gas contains as much as 10% alcohol to raise the octane level to 87. Much of the higher octane gas uses other non-alcohol based additives to increase the octane level. That is probably why some people who use higher octane gas claim they get better mileage over the 87 octane.
Then you can get into such variables as weather, terraine, traffic, weight of the vehicle and cargo, tire inflation, engine tune, temperature, summer vs winter fuel blends, etc It could mean that you will get less mileage during some parts of the year.
...Rich