What is the biggest, best off road tire to switch to from the factory 20" size on a 2012 f 150 supercrew ecoboost? I do not want to do any kind of lift or any modifications to the truck.

 

General consensus is max 33" tire diameter or a 275/60/20 in your case. In terms of tire much depends on your tolerance for noise and tire weight or if you live where it snows all winter.
Myself, I'm riding stock size p rated Toyo Open Country AT2 on lariat 20" no lift or level and I like them thus far. Lost maybe a few tenths MPG vs the old Scorpions. Even in stock size the tires look a lot better than those. Only had them since may and I live on the west coast so I can't comment on winter performance yet. I also considered new BFG AT KO2 but, felt they would look better in larger sizes on a leveled truck and are a heavier tire.
My brother runs michelin ltx at2 up in AK year round on his 2011 with good results. I've heard good things about the MS/2 as well.

Much depends on where you drive.

My 2cents...and welcome to the forum

 

IMO, Goodyear Duratracs are the best all terrain tire in the snow. I had Terra Grapplers on my 06 and I hated them in the snow.

 

Agreed, Duratracks are a good rated tire and seem popular with folks in Northern climates due to great snow and ice performance. They are also light considering they are on the more aggressive of AT tread variety.

 

You can fit a 275x60x20 (33x11) at stock height with no problems. Most of these sizes are P rated except for BFG and maybe Goodyear. I'm running the Toyo AT2's on my truck and they cleared easily at stock height.

 

I'm running 275/65R20 Firestone Destination AT tires on my STX 4x4 (34.1" tire) with no problems.

 

My vote is the Cooper A/T^3. Second is the Toyo A/T. Those seem to give the best mix of price vs performance.

 

While on my last truck which was a Ram I had Cooper HT-Plus. I liked the more aggressive tread and road noise was just as good or better than the Wrangler SR-A that it came with price with install at the time was $690


On my current King Ranch the 20's have a set of 275/65 Michelins that do rub upfront when I'm at extreme turning