# How Automatic Transmissions Work

Reverse Gear
Reverse is very similar to first gear, except that instead of the small sun gear being driven by the torque converter turbine, the bigger sun gear is driven, and the small one freewheels in the opposite direction. The planet carrier is held by the reverse band to the housing. So, according to our equations from the last page, we have:

Ratio = -R/S = 72/36 = 2.0:1

So the ratio in reverse is a little less than first gear in this transmission.

Gear Ratios
This transmission has four forward gears and one reverse gear. Let's summarize the gear ratios, inputs and outputs:

 Gear Input Output Fixed Gear Ratio 1st 30-tooth sun 72-tooth ring Planet carrier 2.4:1 2nd 30-tooth sun Planet carrier 36-tooth ring 2.2:1 Planet carrier 72-tooth ring 36-tooth sun 0.67:1 Total 2nd 1.47:1 3rd 30- and 36-tooth suns 72-tooth ring 1.0:1 OD Planet carrier 72-tooth ring 36-tooth sun 0.67:1 Reverse 36-tooth sun 72-tooth ring Planet carrier -2.0:1

After reading these sections, you are probably wondering how the different inputs get connected and disconnected. This is done by a series of clutches and bands inside the transmission. In the next section, we'll see how these work.