How wide of a tire can i fit on stock 2005 f150 wheels?

 

Don't know what OEM wheel you have? If it's 20's it's a 20x8.5" wide wheel.

The max width would be based on also the height(taller tires allow more sidewall bulge), you can run a 35x12.5" but a 33x12.5"(although some still do it) would not be recommended per tire manufacturers. Most tire websites have a fitament guide, and I would suggest using the "measured wheel width" for the optimal wheel/tire size, but you can use the min-max for the recommended range.