Question

A new car has factory installed tires that are P205/60R16. The owner wants to a little bigger and opts for the set of P215/75R15. How fast is the car actually going if the speedometer is at 80?

Answer 1

The car is actually moving at a speed more than 80 miles per hour

Explanation:

Theoretically, since the speedometer of the car is at 80, it means the car is having a speed of 80 miles / hr. The car should move by a distance of 80 miles in an hour.

But, practically changing the tire size can affect the speedometer accuracy because larger tire covers more ground in each complete revolution. So, the actual speed is higher than the speedometer reads.

Example

Suppose a car comes with factory-installed tires which are 21.8 inches in diameter. It means that the circumference of each tire is 68.5 inches. Now if we replace the old tires with new tires having 24.6 inches as diameter then each new tire has a circumference of 77.3 inches. This means the car travels almost 10 inches more with each complete revolution. This can tremendously affect the reading on the speedometer. The speedometer will now indicate a speed which is too slow by almost 13%. When the speedometer reads 60 miles/hour, the car will actually be traveling at a speed of 67.7 miles/hour.