"En gearing udregnes som (27*antal klingetænder)/(antal kranstænder) og er et enhedsløst tal, der lidt intetsigende udtrykker gearets størrelse eller "tyngde", om du vil"
De "27" er dog ikke grebet helt ud af luften, og geartallet angiver gearingen i tommer på en væltepeter, hvis man kigger til Sheldon Brown :
One of the three comprehensive systems for numbering the gear values for bicycle gears. It is the equivalent diameter of the drive wheel on a high-wheel bicycle. When chain-drive "safety" bikes came in, the same system was used, multiplying the drive wheel diameter by the sprocket ratio. It is very easy to calculate: the diameter of the drive wheel, times the size of the front sprocket divided by the size of the rear sprocket. This gives a convenient two- or three-digit number. The lowest gear on most mountain bikes is around 22-26 inches. The highest gear on road racing bikes is usually around 108-110 inches. Unfortunately, the handwriting is on the wall for all inch-based measurement systems.
The second-oldest style of bicycle, the successor to the "bone shaker," and the predecessor of the modern "safety" bicycle."
Before the use of chain drive, bicycles had direct drive. The cranks were directly attached to the hub of the drive wheel. The larger the wheel, the farther the bicycle would move with each turn of the pedals. The diameter of the drive wheel determined the gear of the bicycle. The larger the wheel, the higher the gear.