'06 FX-4 scab
tires are gone & have to do something
Questions....

there seems to be more selection for 17 & 18 inchers than for OEM 20s
any dissadvantage in dropping down in size and going with larger tires
specifically Brake heating etc ???????????

 

No disadvantage as long as you stay close to the same overall diameter on the tires, so the speedo will not be off. That is, unless you have a tuner and you can sdjust the speedo.

 

Here's a couple of stock oem sizes for you.
275/65r18
265/70r17
On stock oem rims for that tire size.
With squish accounted for there really shouldn't be any need for speedo adjustments.
The same brake rotor size is used from 17-22 rims

 

The advantages of the 17" wheel as opposed to the 18" wheel according to Ford is about 500lbs greater hauling capacity... Everything else being equal...

 

That is only because the load rating on the OEM tires is higher on the 17s than on the 18s or 20s... nothing else is changed with the truck.

 

You will have a better tire selection at much better prices if you drop down.

 

Tires are not exactly cheap when you have to buy the wheels to fit them.

 

I'd suggest adding a 2 or 2.5 inch Autospring leveling kit and some aftermarket 18x9 inch wheels with 5 to 6 inches of backspacing and some 33 inch tires like a 305/60-18 or 325/60-18.