The manual/instructions that came with the computer should have a list with tire sizes and the corresponding number you'll need to enter into the computer when setting it up.
If you don't have the manual/instructions they're probably online. What computer did you get? Brand and model.
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The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.