Gear ratio can now calculate speed at given cadence.

cyclinglib/gear_ratio.py

@@ -1,7 +1,20 @@
+from cyclinglib.speed import Speed
+
+
 class GearRatio:
 
     def __init__(self, front_teeth: int, rear_teeth: int):
         self._gear_ratio: float = front_teeth / rear_teeth
 
+    def speed_at_cadence(self, cadence: int,
+                         wheel_circumference_mm: int) -> Speed:
+        """
+        Cadence is specified in revolutions per minute.
+        """
+        revolutions_per_second = cadence / 60
+        wheel_circumference_m = wheel_circumference_mm / 1_000
+        return Speed(wheel_circumference_m * revolutions_per_second *
+                     self._gear_ratio)
+
     def __str__(self) -> str:
         return f"Gear ratio: {self._gear_ratio}"