I have an 04 SCREW with the 3.55 rear end. If I upgrade to the FX4 18" rims will it affect anything. I noticed that all the FX4's with 18" wheels have the 3.75 rear end.

 

No, just get the 18" rims and stop worrying about it.

PS: Its a 3.73 rear end.

 

My bad, 3.73. What is the difference between the two sizes?

 

0.18

 

To give you a more straight answer, yes you will notice the gear change when you put the taller tires on. I had the stock 30 inch tires on my two wheel drive regular cab. I went to the slightly taller 265/60/18 Lariat rims. I can notice the taller gearing. But that is only a .6 inch increase. The 18" off road tires (Rugged Trail, etc.) are 31 to 32 inches depending. That was enough to discourage me from using Lincoln wheels/tires, or other large ones.
That is because I have the 4.6 engine. With the 5.4 you will not notice it as much. The speedo will be off about 5 percent. It will read lower than actual.

Should you do it? Probably. The loss in power on launch is not that big a deal. The dealership can set your speedo correctly, or you can use a tuner.

The 18 inch does look a lot better in my opinion.

If you tow, you will lose a bit of your tow rating, both because of the high gearing, and the extra weight of the wheels and tires.

Your end gearing with the tallest stock 18 combo will end up being the equivalent to 3.31

That what you wanted to know?

 

Answers my questions. Thanks for the info.