Combined sales tax is a state rate plus whatever county, city, or special-district rate stacks on top, so it varies by exact location, not just by state. Alaska, Delaware, Montana, New Hampshire, and Oregon levy no statewide sales tax, though some allow local option taxes.
Many states exempt or tax at a reduced rate certain categories, such as groceries, prescription drugs, or clothing. On a vehicle purchase, whether tax applies before or after a trade-in credit depends on the state — pair this with the Auto Loan calculator when financing a purchase.