You'll be told to spend 35% of your income, or 10% of gross monthly, or half your salary, or one year's salary. These are quoted as if they're interchangeable. On the same income they land nearly four times apart. Here's what each one actually says for you, and which is worth following. You can also run it backwards — enter a car price and see what salary each rule demands.
The rules aren't measuring the same thing, which is why quoting them side by side without saying so is misleading.
If you're financing, 20/4/10 is the one worth following: 20% down, no more than four years, and total car costs — payment plus insurance plus fuel plus maintenance — under 10% of gross income. It's the strictest of the common rules and it's strict for a reason. The 20% down payment keeps you from going underwater, the four-year term stops you paying interest on a depreciating asset for most of a decade, and the all-in cap prevents the classic mistake of budgeting for the payment and forgetting the insurance.
If you're paying cash, the salary-multiple rules make more sense, and half your annual salary is the sensible ceiling for most people. One year's salary is defensible only if you have no other debt, a full emergency fund, and you intend to keep the car a very long time.
The 35% rule sits in the middle and works reasonably as a quick sanity check on the sticker price before you look at financing at all.
Every rule here is built on income alone, which is their common weakness. They don't ask about:
Treat whichever rule you pick as an upper bound, then subtract for anything above that applies to you.