Max trailer tow REQUIRES the 3.73 axle. HD package also requires it, so if you already have it ,you can't add it.

To answer the OP. Rear axles for the trucks vary from 3.15 to 3.73 (not counting the 4.10 with the Raptor or FX4). We don't know which axles Ford used for EPA testing but would assume the highest 3.15 or 3.31. So with the 3.73 I would expect maybe 1 or 2 mpg lower but then again maybe not. The 3.73 will be better off of the line, so city mileage may actually be better because it will take you less gas to get moving. Hwy mileage will be a little lower.
If you commute over 100 miles a day you would want the 3.15 or 3.31 for max hwy mpg. Me, personally I went with 3.73 but will drive 65-70 instead of 75 to make up the difference. In other words, the 3.73 will run a slightly higher rpm than the the other axles, therefore burning slightly more fuel. By slowing down a few mph, you can run the same rpm and achieve the same mpg