The biggest tire that can be put onto a 2018 stock Chevy 1500 is a 31″ diameter tire. Its dimensions are 265/70-17 and it would fit comfortably without trimming. The normal tire size is 31″ 255/70-17 but can range based on what you want. Lifting the truck can allow more room for larger tires. For Top, Chevy Silverado Tires click on the link.