Wheels and tires

I just bought a 2014 Tundra 1794 Edition. The wheels are 20 inch over size and the tires stick out past the fender. The tires are all a mish-mosh of off road type tires. I intend to replace these with normal rims and tires. My questions: Can I go to 18 inch rims and if so:
1. Will the 18 inch ride smoother than the 20 inch?
2. Which one will provide the better gas milage??

I don't do off roading. I only need the 4 wheel drive for weather and pulling large boat up the ramp.
Thanks in advance for your advice-Sam

 

1. Yes
2. Stock size, P-rated tires will provide the best mileage

 

I will have my 18" wheels for sale before to long. They are aftermarket but they are 18x9 +1 I would take 600 for them when I get my new wheels set up. But I'm in Indiana not sure where you are.

Here are my wheels

 

Got any pics of how far they stick out past the fenders? If the op isn't interested in them, I possibly could be.

 

Not sure how much shipping would be but they are out by like 1" but I'm also running 12.5 wide tire so I dont think a 10 wide or something near stock would be to bad at all

 

Not the best pic but you can kinda see how far.