This web site has some great automotive calculators that you can input your data and see the results, so you don't have to rely on goofy friends that probably don't have enough sense to buy a Ford.

Automotive Calculators offer to you by Ford Trucks Enthusiasts | www.ford-trucks.com

OK, first we have to know what your stock tire size was and your stock gear ratio. I'll guess and say you had 33s and you have 3.73 gears.

Going from 33s to 35s is like taking out your 3.73 gears and installing a 3.51 ratio.

To get back to stock gear multiplication you'd need roughly a 3.95 ration. 4.10 would pretty close, and in addition give you a little more gearing.

If you went to 40 inch tires, you'd need a 4.52 ration to get back to stock.

5.13 is not too high. Running a 5.13 ration with 40s would be like running 4.13 gears with 33 inch tires. That is pretty close to a 4.10 stock ratio. With that the factory says you can tow a bit over 10k.

One thing to be careful about; Most of the larger size tires are D rated, not E rated. If you're planning on towing heavier loads be aware of this and check the actual load rating of the tire.

The 5.4L and 6.8L gas engines aren't really sensitive to gear changes when it comes to fuel mileage. Members report about the same fuel mileage unloaded after going from 3.73s to 4.10s or even 4.33s. What will kill your fuel mileage is the lift.