For a more precise calculation, the size of the tyres used on 16" wheeled Stridas is 16 x 1.50 or 40-305 in ETRTO system.
According to the tyre circumference cross reference table on the instruction manual of my cyclocomputer, the actual circumference of a 16 x 1.50 (40-305) sized tyre should be 1185mm. Then, the actual diameter should be 1185 ÷ 25.4 ÷ π = 14.85".
Therefore, the actual gear ratio of Strida 1, Strida 2, Strida 3, Strida 5 and Strida LT is 14.85" × 100 ÷ 30 = 49.5 gear inches.
In the case of Strida Mini, the size of the tyres is 14 x 1.50 or 40-254 in ETRTO system. The circumference of the tyres is 1020mm. Then, the actual diameter is 1020 ÷ 25.4 ÷ π = 12.78". Therefore, the actual gear ratio of Strida Mini should be 12.78" × 88 ÷ 30 = 37.5 gear inches.
However, Strida Mini uses shorter crank arms compared with a normal sized Strida with 170mm crank length so the gear ratio feeled by the rider will be higher than the actual gear ratio. 
On Strida SX with Innova tyres, the size of the tyres is 18 x 1.35 or 35-355 in ETRTO system. The circumference of the tyres is 1325mm. Then, the actual diameter is 1325 ÷ 25.4 ÷ π = 16.6". Therefore, the actual gear ratio should be 16.6" × 100 ÷ 30 = 55.3 gear inches.
On Strida SX with Schwalbe Kojak tyres, the size of the tyres is 18 x 1.25 or 32-355 in ETRTO system. The circumference of the tyres is 1315mm. Then, the actual diameter is 1315 ÷ 25.4 ÷ π = 16.48". Therefore, the actual gear ratio should be 16.48" × 100 ÷ 30 = 54.9 gear inches.
On Strida SD and the MAS Special Edition, the actual gear ratio in low gear is 14.85" x 80 ÷ 30 = 39.6 gear inches and the actual gear ratio in high gear is 14.85" x 80 x 1.65 ÷ 30 = 65.3 gear inches.
On Strida equipped with speed-drive and 18" wheels (with Schwalbe Kojak tyres), the actual gear ratio in low gear is 16.48" x 80 ÷ 30 = 43.9 gear inches and the actual gear ratio in high gear is 16.48" x 80 x 1.65 ÷ 30 = 72.5 gear inches.