What is the biggest tire I can install a on stock 20 inch wheel.

 

On what? How wide is the wheel? Biggest as in tallest or biggest as in widest? Do you care about clearance?

 

I have an 07 f150 fx4 with a 2 inch leveling Kit. I wanna keep the stock wheels but have the biggest tire possible. I have 275x55x20 now.

 

Per the specs for our HP10253 2" kit, you'll want to go with a maximum 35" tall tire on a 4WD truck (33" for a 2WD). There seems to be anecdotal evidence that something bigger can work, though it's not recommended.

 

Go to Tire Rack and they will provide the specs on tires that include the recommended rim width for given tire.
My stock 17" 7 1/2" wide rims have 285/70x17s on them and the rim is the minimum width recommended for that tire.

 

It's been my observation in 30 years that a tire can be 1.5-2.5" wider then the rim. Like my 20x10 has a 295/40/20 which technically is a 11.6"wide tire and looks almost like a perfectly straight side wall. I think a 305 would of looked a little better though at 12.06" wide. On a truck I had with a 16x10 had a 37/12.5/16.