What is the circumference of a car tire?

What is the circumference of a car tire?
Check the sidewall markings. For example, it might say: "205/55 R16 91H" For circumference, you need the first three numbers. (The last number is the weight rating, the last letter is the speed rating. The "R" means radial, which all modern tires are.)

205 is the width in mm
55 is the ratio of the sidewall height in percentage of the width. (IOW: 55% of 205 mm)
16 is the wheel diameter in inches

First convert the tire width to inches:
205 / 25.4 = 8.07 in
Then get the sidewall height:
8.07 * 55% = 4.44 in
The diameter of the tire is the wheel diameter plus two sidewall heights:
16 + 4.44 + 4.44 = 24.88 in
Finally, circumference is 2 * Pi * radius, or pi * diameter:
3.14 * 24.88 = 78.1 in

In one formula, the above would be:
( (205/25.4 * 55/100 * 2) + 16) * PI()

Also, there are many online calculators that will calculate all this for you.