A "rollout" is often more accurate than the tables (by a few percent, usually). The wheel is rolled along a flat surface and the distance measured. Those numbers (usually somewhat near 2000 for a 700c road bike) are the circumference in mm.
The bike computer counts revolutions using the magnet and sensor, and times these. So the circumference tells it how far each turn of the wheel moved, giving distance and speed.
This is easier with two people. roll the wheel until the label or the valve stem is at it's lowest point. Mark the ground there--I use a piece of tape. Roll the bike forward -- I do two full revolutions. Mark the ending point.
Measure with a tape measure. To convert inches to mm, use a google search like this example: 160 1/4 inches in mm. It shows 4070.35. Then divide by two for the two revolutions: 4070 / 2 = 2035mm.
It works pretty well with just one revolution, a bit less accurate, but likely good enough. With your 20 inch tire, two or three revolutions should be easy to do.
I've seen recommendations to use your weight sitting on the bike to press down on the tire. I don't usually see much of a difference, and this isn't too easy to do.
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The table shows 1580 for the 20 x 1.75. If you get 1524 for example, that's 1524/1580 = .964 or a little more than 3% difference.
Last edited by rm -rf; 11-10-21 at 03:29 PM.