Changing from 18" wheels to 20" what should I do.
 

I'm changing my 2004 18" Lariat wheels to the new 20" wheels, should I make any adjustments to the engine to compensate for the bigger wheels and if so can this be done by the dealer. I dont want any shade tree performance mods added to my vehicle. 5.4L

The size of the wheel itself doesn't matter. What is the diameter of the tire you are putting on the wheel?

Not trying to hijack the thread but my question is similar. If I go from the OEM 18 275/65/18

What do I need to go to in 20"?

did u just purchase the wheels from the dealership? If so how much for rims and tires?

Just to use an example, the Goodyear LT275/65R18 oe tire is 32.1" in diameter per their website. If you went to something like the BFG AT in LT285/55R20, per BFG that is 32.5" in diameter, not a big difference. If you go much larger than that you will need to install a leveling kit or lift.
I am also making the assumption you are talking about a 4x4.

Yes I am talking about a 4x4 but I don't use it for off roading. 4x4 is mainly for traction in the winter when I need it.

The tires are 275/55-20 Pirelli scorpions and if anyone is interested I'm looking to sell the 18" Lariat rims including tires with about 95% tread left BF Goodrich

And to achap , no I didnt purchase them from a dealer they were takeoffs from a 06 King Ranch the dealer wants $700 a piece just for the rims, I got a good deal on ebay about $1200 for the tires and rims including shipping.

I have 20" rims (wrapped with toyo 35/12.50's. That 5.4 has no troubles with them. I do plan to get a programmer soon, to help with the shifting.

shouldn't have to just put them on.