I just bought my first ford, an 04 f-150 lariat supercab 4x4
it needs new tires currently there are 275/65 r18 on factory rims what bigger sizes could i get away with running ?

 

You have several options. If you are gonna leave the truck completely stock and not lift or level it, I think that a 285/65-18 (33x11.50) is a great choice. A 285/60-18 (32x11.50) or a 33x12.50-18 also looks good. If you install a leveling kit, you can fit a 305/65-18 (34x12.50) or a 305/70-18 (35x12.50) using you oem 18x7.5 inch wheels. If you want to install a leveling kit and wider aftermarket wheels, I'd stick with a 33 inch tire.