okay, so I'm a second gen owner, but my dad has a third, he had put 20" rims on his truck and now realized before winter that tires are really expensive, so my question is, will 16s off a second gen fit?

 

No they have to at least 17s are they will not clear the brake cailber

 

+1
17" is required.

What kind of savings are you seeing between the sizes?
I still don't understand how a 35x12.5x20 tire is more expensive than a 35x12.5x17 when the 17" tire contains more rubber. Pay more, get less tire. Crazy.

 

Less consumer demand for 20's equal less sold as replacements hence higher price.

 

With all the vehicles being sold with 18"+ wheels, I never would have guessed that, but you seem to have nailed it. Doing some research into the matter, as late as 2010 nearly all of the 10 most popular tires in the US by size were 16&17". Explains why a 35x12.5x17 is on average $50-60 per tire cheaper than a 35x12.5x18.

 

Lower mass production of tires for larger wheels is one, also the "BLING" factor of 20" and larger wheels contributes to it A LOT. Guys who just have to have big honkin' wheels will usually pay the premium price for them and the corresponding size, rubber band-like tires that go on them...

My advice to the OP, find some low cost 17" steelies or even used 17" rims and go get some 17" tires relegated for winter use...

 

18" is a newer standard size, especially by Ford. So, they are very expensive. Last time I checked, 18's were more expensive than 20's.

Tires are expensive no matter what and it depends on what kind of rubber you are looking for. I spent $700 for my rubber(275/60/20 LTZ's)3yrs ago. I can't imagine they would be more than $850 now.
Eitherway, I have always oredered from Discounttiredirect or Tirerack because not one local competetor could come close to their price after shipping and mount/balance fees. Plus, no tax on out of state ordered items.

 

when i was running stock 20s i bought a set of brand new 20 inch tires for 300 plus another 100 to ship

 

Local quote of 275/60R20 BFG Long trail T/A's mounted, lifetime balance/rotate, road hazard and front end alignment. $1042.77. Average price for a 20" tire with the proper load capacity for a truck is around $200 a tire.

Just an off topic note, my OEM Badyears have 55K on them and still haven't hit the wear bars yet, but I won't run them thru another winter.

 

My OEM tires still had plenty of tread left at 55k as well, but I let the body shop that replaced them have them. I don't wish those tires on anyone. I understand making the rubber compound harder for longer mileage at the expense of handling, but where's the limit? Those tires were meant to go a very long distance in a straight line LoL