Gear ratio refers to the number of teeth or the size of one gear in a gear set in relation to the other. The gear set in the top illustration indicates gear reduction. The drive gear turns twice for every one revolution of the driven gear. Its gear ratio is 2:1 and increases torque. Gear reduction is used for 1st through 4th gears in the transmission in the chart below. Notice the first gear ratio in the chart is 4.027:1.
The gear set in the second illustration indicates overdrive. In this set the drive gear turns once for every two revolutions of the driven gear. This type of gear ratio increases speed over torque and is used for 5th and 6th gear. The 5th gear ratio in the chart is 0.852:1. Notice that the 1 represented by the driven gear is assumed and left out.