Is there an advantage to one over the other, or is is just looks. I will probably go with an after market rim some day anyway. I just want to know if paying for the 20" rim give me something else other than looks.

 

18" rims have alot more options for tires, the tires are usually cheaper, and typically more rims to choose from.

20" inch rims at the moment are the exact opposite.

I started my search with a specific backspace range in mind and wanted 18 inch rims. I could not find an 18" rim with the backspacing I wanted but found it in a 20". Shopping for rims can be a bitch unless you dont care how much the tires will stick out the fenders.

 

20's will ride a bit more firm and more controlled, 18's will ride a bit nicer so if you're on rougher roads or going off-road are better, more sidewall is more protection for the wheels.

 

I like the looks of more tire and less rim on a truck...they both look good but the 18's IMO for a truck look a bit more rough...if that makes sense...you cant go wrong in either case.

 

Choose your wheel (OEM or after market) based on looks. Most people would not notice a difference in ride between the same tire same OD on an 18" wheel vs. a 20" wheel.

 

If you will use your truck offroad at all, you want the 18' rims. The P series tires that are prevelant for the 20" rims do not have the sidewall strength and puncture resistance that you get with LT series tires in the 18". Ther are very few LT tires for 20". About all you see on F150s in South Texas, in cactus and mequite country, are BFG All Terrains and a very tough Michelin. You have to jack up a truck to chage a tire flattened by a mesquite thorn only once, especially at night or in the rain, to appreciate the utility of the tougher tires. They do reduce your gas mileage a bit, but that is a small price to pay if you are leaving the pavement.

 

I like my 20's just because now I can say "I got Dub's" hah..

It all depends on your use. I'm not crawling rocks and abusing my $44,000 truck so I'm not worried about the tires. The most offroading i'll be doing most likely is on sand at the OBX.

 

20" and 22" are more difficult to balance. I have factory 22"s on a Limited and they have been a bit more difficult to keep smooth. Both sets I've had. They also have temporary flat spots sometimes after sitting and will smooth out after 10 miles or so. The tire shops don't like the larger diameter rims due to balancing issues.
All that said and I am cosidering swapping from 18s to 20s on my next F150.
Just like the looks but will give the 18s a shot first.

 

You have greater towing capacity with 18" wheels versus 20" wheels.

Which trim level are you considering with the wheels?

 

Does having 20s effect your gas mileage, performance, or towing worse than having 18s? Reason I ask is I have an '07 with 18s currently and the 4.6L engine but I'd like to upgrade to 20s. Will this kill my performance and towing?

 

Not as long as you get a tire that has the same outside diameter (OD) as the 18's you currently have. The wheels will be slightly heavier, but it shouldn't be noticeable on a 6K lbs truck.

 

Just guessing but I'm thinking he might be trying to decide between the standard 18" wheel & tire combo and the 20" wheels and Pirelli's that come on the FX4 sport

For me, its a no brainer. Between construction sites, country roads, hunting, cutting wood and having fun the 20" wouldn't stand a chance. For my purposes I'd consider the 17" that come on the STX and put an even taller/wider tire on as long as the offset still works and the rims clear the rotors.