Use your Garmin Nuvi Mount that may already be fastened somewhere in your car to also hold your cell phone.  These mounts include a cradle that opens from 2.25 to 3.40 Inches.  Also included is an adapter that fits the Nuvi 17mm ball.

The mega grip holder is designed to securely hold larger mobile phones, smartphones, and PDAs such as BlackBerry or iPhone. It's the perfect companion product if you're using a GPS application on your phone. Universal style holders, unlike custom fit holders, will work with your phone regardless of whether you have a protective skin or case on it. It's the most versatile mount solution for your phone.