Gas mileage can be very elusive, and fluctuate due to seemingly minor changes. I assume your driving habits have not changed, so here are a few suggestions. Probably no single item is causing your reduction in fuel mileage, but a combination of several factors.
Different tire models and manufacturer can have a different diameter even if they are they same size. This can induce a small amount of speedometer/odometer error that may account for some of your mileage loss.
Also, different tread patterns and tire design can increase the roll resistance and make the tire use a little more gas.
Different gas from a different pump may account for some of the variances as well as where you buy your gas. Most 87 octane gas contains some Ethynol (alcohol) which gets about 25% less mileage. Different brands blend different amounts of Ethynol to their gas. Check the amount of ethynol in the gas you are using and look around for an 87 octane that contains less ethynol. Your gas station may have changed how they blend their gas?
The other big variable in gas mileage is the traffic. Now that school is starting, traffic is getting heavier and the more stops/starts or slow-down-speed-up cycles in your daily commute, the more gas you will burn and that can be significant in the Sport Trac due to the heavier weight. Also, consider that many people may be on the roads in your area who are on vacation and that may contribute to a slight increase in traffic. Just catching a few more traffic lights can burn more gas, and you may not even notice the extra traffic, or that you have caught a few more lights, or the lights stay red longer, etc.
Also, the weather can play a big role in mileage. Cooler temperatures may require a little more gas for the engine a little to warm up, while really hot weather lowers the engines power.
Rain and snow will increase tire slippage and reduce mileage.
Way too many causes than can be listed, but it could also be an engine sensor has gotten a little out of tollerance but not enough to trigger a CEL, but coupled with the other factors listed above may be eating a little more gas than it did before?
...Rich