I figured out the "code".

Tire diameter in inches * PI (3.1416)

Your tire- 40mm

622MM (rim)

622+40+40

=702mm (Diameter of the tire)

1mm = .03937"

702*.03937= 27.638"

27.638" * Pi (3.1416) =86.8"

*10 = 868

868 is the "magic number for you!

It appears since your tires are listed as 42 AND 40 MM, 40 would probably be the number to use in your initial calculations. (to allow for the slightly smaller actual size)

Since this is the 50+ forumn, I had to make sure you're "old enough"

I've BEEN wearing bifocals for 49 years!!!