To get accurate results from a speed sensor, input your wheel circumference.
An accurate speed sensor can also improve the accuracy of GPS Bike Computers, especially in tree cover or near buildings.
You can find tire size on the side of your tires. 
Tire circumference may vary depending on tire pressure. 

Properly inflate your tires for the most accurate results.

If there is no relevant model on the tire, the circumference of the bicycle wheel can be measured (unit: mm), which is the wheel diameter that the computer needs to set.