An automatic gear shift system for a bicycle having an electronic device for changing gear ratio, the system comprising: at least one sensor operable to sense one or more of bicycle speed, pedal cadence, and pedal power; a control system having a processor operable to execute program instructions to; receive data from the at least one sensor; determine a condition change in the data and/or in a calculated value based on the data over a predetermined interval; compare the condition change to at least one threshold value associated with the at least one sensor and/or the calculated value stored in a memory of the control system; and send a command to the electronic device to execute a change in gear ratio if the condition change exceeds the at least one threshold value, wherein: the at least one sensor includes at least one sensor operable to determine external conditions to the bicycle; and the at least one threshold is determined in relation to the external conditions, characterized in that the at least one sensor includes a GPS receiver and the external conditions includes whether the bicycle is traveling uphill or downhill, and the processor of the control system is operable to execute program instructions to: evaluate data from the GPS receiver to determine an upcoming change in grade; and execute a change in gear ratio prior to the change in grade. at least one sensor operable to sense one or more of bicycle speed, pedal cadence, and pedal power; a control system having a processor operable to execute program instructions to; receive data from the at least one sensor; determine a condition change in the data and/or in a calculated value based on the data over a predetermined interval; compare the condition change to at least one threshold value associated with the at least one sensor and/or the calculated value stored in a memory of the control system; and send a command to the electronic device to execute a change in gear ratio if the condition change exceeds the at least one threshold value, wherein: the at least one sensor includes at least one sensor operable to determine external conditions to the bicycle; and the at least one threshold is determined in relation to the external conditions, characterized in that the at least one sensor includes a GPS receiver and the external conditions includes whether the bicycle is traveling uphill or downhill, and the processor of the control system is operable to execute program instructions to: evaluate data from the GPS receiver to determine an upcoming change in grade; and execute a change in gear ratio prior to the change in grade. receive data from the at least one sensor; determine a condition change in the data and/or in a calculated value based on the data over a predetermined interval; compare the condition change to at least one threshold value associated with the at least one sensor and/or the calculated value stored in a memory of the control system; and send a command to the electronic device to execute a change in gear ratio if the condition change exceeds the at least one threshold value, wherein: the at least one sensor includes at least one sensor operable to determine external conditions to the bicycle; and the at least one threshold is determined in relation to the external conditions, characterized in that the at least one sensor includes a GPS receiver and the external conditions includes whether the bicycle is traveling uphill or downhill, and the processor of the control system is operable to execute program instructions to: evaluate data from the GPS receiver to determine an upcoming change in grade; and execute a change in gear ratio prior to the change in grade. the at least one threshold is determined in relation to the external conditions, characterized in that the at least one sensor includes a GPS receiver and the external conditions includes whether the bicycle is traveling uphill or downhill, and the processor of the control system is operable to execute program instructions to: evaluate data from the GPS receiver to determine an upcoming change in grade; and execute a change in gear ratio prior to the change in grade. evaluate data from the GPS receiver to determine an upcoming change in grade; and execute a change in gear ratio prior to the change in grade.