When I enter the following to the Sheldon gear calculator:

Tire: "20 X 1.75 / 44-406 / BMX tire"

Chainring: 52T

Cassete: 11T

I get 88.3 gear inches.

I presume that the formula is: gear_inch = (52 / 11) x 18.68"

Where does the 18.68" constant comes from? Is it part of the specification of the rim and/or tire? 406mm is about 16" so is the extra 2.68" the contribution of the tire to the diameter?

(in my case, I plan to use Schwalbe Marathon Plus 20 x 1.35" (406)).

Thanks,

Kam