i have an 04 FX4 with a 3" leveling kit on the front with a 1" on the rear and was curious as to what the biggest size tire i could put on is? Possibilities open for a 20" rim . Looking at moto metal 951 rims?

 

you might be able to stuff a 35" tire in there if you use a wheel that has backspacing close to your stock wheel. my 951's are 20x9 with +30 mm offset= a little over 6" of backspacing. I could fit 33's no problem so i'm pretty sure you would be able to use this wheel with a 35" tire no problem. I'm a 4x2.

 

You can get Moto Metal 951 wheels in 2 backspacing (well at least on the 18's). they come in 4.5" or 6" backspace. I run a 33" with 4.5" backspace. If you run a 6" backspace your tires will be tucked in significantly more (about like stock) and you can get away with a bit taller tire, probably up to 35".

I had some rubbing on the fenders and valance. Easy fix with an angle grinder and cutoff wheel. Now I have almost no rubbing in any on or offroad situation. If you run the 6" with a bigger tire, you may get rubbing on the steering/suspension components at full lock. Can't fix that, just have to deal with it.

The 2 different backspacings will give you a significantly different look. I like the way my tires stick out and look big even though they are only 33". Better for my gears and less rubbing, then again I can get mud and stuff thrown up on the truck easier.

Hope this helps. Look at other trucks and figure out what kind of look your going for. Backspacing plays a big role in the look/fit of tires.

 

You can go up to a 35x12.5 on a stock wheel or one with close to stock specs on a leveled 4x4.

 

I agree. You are gonna have to find an aftermarket wheel with specs almost identical to the oem wheel's (7.5 inches wide with 5.5 inches of backspacing) to fit 35's.