There is a lot of discussion on this topic at f150forum.com, and previously I spent a lot of hours reading up on this, and my conclusion was 275/60R20 on stock should be fine--above that and without lift or level and you may experience some rubbing. It seems a lot of guys in those threads are swapping out their shocks for Bilstein 5100's where they can make minor adjustments to the ride height all the way to leveling off...
I personally like the looks of the 275/60R20 over the stock 275/55R20, that little bit of extra sidewall height just jumps out to my eyes. A lot of RAM 1500's come stock with 275/60R20's, so park next to one of those for a visual comparison.
One thing to keep in mind, if you upsize your tires you will also need to upsize your spare tire, and our spares are on 18" wheels without a lot of room to store a tire that is much bigger without doing some modifications to provide for that.
Tire size modifications also should have one recalibrating speedometer, and from my research this is most easily achieved with FORScan, and may also want to use FORScan then to modify the TPMS (Tire Pressure Monitoring System) thresholds as well.
@Bedgir ... I saw in another post that you have 3.31 gears. Upsizing your tires like that would have an effect on your rearend gears as if you changed them from 3.31 to like ~3.17...
As far as gearing goes, if you have a 3.55 rearend and 275/55 R 20 tires (32 x 10.83 x 20), changing to 305/55 R 20's (33 x 12.01 x 20's) would be the same as if you kept your current wheels and tires but changed your rearend gears to 3.41's.
Also, just only doing a level on the truck with these tires being wider could have them rubbing on the crash bars, but I am by no means an expert on this subject. You may want to do some research on f150forum.com where there are a lot of posts on tire sizing and lifts and levels.
I do like your choice in Ridge Grapplers, and I may go that route when I get around to it. I was running LT Duratracs on my 2010 for many years and loved how those tires handled on-road and off-road.
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