There are probably better phone holders out there that use the weight of the phone to hold it tight but I just thought that one using gears this way would look cooler.

Please note that this is only the part to hold the phone, you'll need to add something to attach it safely in your car. See below.

The supported width range is limited to around 35mm, by default from 60mm to 95mm. The size can be customized but the range is fixed, so you can change it to support landscape oriented phones but not to support both portrait and landscape oriented phones.
The supported depth is by default max 12mm. You can change this with the clamp depth parameter in the customizer.

Printing instructions:

• The clamps and the frame need support, but you may want to prevent support being added inside the rails. It shouldn't be needed there and might be hard to remove resulting in too much friction.
• The pins can be printed vertically without support. If they break too easilly though you may try to print them horizontally with support.

Mount

A custom mount has to be added to safely mount the holder into your car. Either:

• Use a 3D modeling tool to attach a mount, either directly on the frame or use the frame_with_mount_base file. This adds a 20*20 base with 1mm rounded edges which may make it easier to attach a mount
• Download the openscad file and edit the module mountbase.
• I quess that, as a last option, you can always physically glue or otherwise attach the frame to some mount.

Customizer

Use the customizer if you want to adjust some parameters, for example to hold a landscape-oriented phone. If you have troubles using the customizer, just install OpenSCAD locally and open phoneholder.scad with it. Use Render to render the object and Export as STL to export it to STL.