* Check your owner's manual: This is the most reliable source. It will specify the lug nut size and other important wheel information.
* Look at the lug nuts themselves: The size is often stamped on the nut itself.
* Check your wheel's specifications: The wheel will have information stamped on it that may specify the lug nut size.
* Use a lug nut gauge: Auto parts stores sell lug nut gauges that allow you to determine the size quickly.
* Consult a parts store: Provide them with the year, make, and model of your truck, and they can look up the correct lug nut size.
Without knowing the specific year and model of your C1500, I cannot provide the correct lug nut size.
