The Gear ratios tab:

This tab lets you to adjust the gear ratios of the transmission. Select a transmission type from 'manual', 'automatic' and 'DCT (Dual clutch transmission)'. Select the number of forward speed gear ratios for your gearbox. Direct drive gear is usually the gear with a gear ratio of 1.00. It is also the gear on which the vehicle gets its top speed. It is usually one or two less than the top gear. Final drive is the gear ratio of the differential.

On the right, there is a large graph containing multiple curves ploted against force Vs. speed. The red line in this graph is the traction limit of the driven wheels. If driving force on the wheel goes above this red line, wheels would spin out. Blue curve is the resistive force on the vehicle due to air drag, rolling resistance and other losses. If driving force goes below this line, the vehicle wouldn't accelerate.

There are some black curves on this graph. These are driving forces of all the forward gears. There is also a gray curve for the reverse gear. These curves are simply torque curve of the engine multiplied by gear ratio and final drive ratio and divided by the radius of the wheel. There is a spot (a tiny circle) on each of these forward gear curves. This is the point where engine gives out maximum amount of power. While adjusting the gear ratios, the maximum power point should lie on the blue curve. This would give maximum possible top speed. It can be done by adjusting the value of final drive gear ratio. Then adjust the first gear ratio so that its curve touches or partially goes above the red line, because a little wheel spin feels nice during acceleration. All of this adjustment can be done automatically using 'Auto-adjust' button.