Redline is not a driving aid. It is a record of driving you have already done.
Start the trip, mount the phone, then set off. Everything the app records happens without you touching it.
Do not touch your phone. No statistic in this app is worth looking away from the road for. Distance, speed and G-force are all still there when you stop.
If you use Android Auto, the car display is deliberately minimal and updates about once a second — slowly enough to glance at, not to read. It exists so you are less tempted to reach for your phone, not to give you something to watch.
The sprint timer and segment timing measure hard acceleration.
Use them on private land, a closed circuit, or a legal test venue. Do not use them to time yourself on a public road. Speed limits apply regardless of what any app is measuring, and a timed run on a public road endangers other people.
Redline has no global leaderboard, deliberately. You are never competing with strangers, so there is never a reason to drive harder than is safe in order to move up a list.
Use a proper mount. A phone loose in a cup holder or on a seat produces poor readings — and becomes a projectile in a collision.
Keep the mount clear of airbag deployment zones and out of your view of the road. In many countries an obstructed windscreen is itself an offence.
Recording uses GPS and sensors continuously and will drain the battery faster than normal. On a long drive, use a charger. Phones in direct sunlight on a windscreen mount can overheat and throttle or shut down.
Sensors can be wrong. If a figure looks implausible, it probably is — a bad GPS fix, a knocked mount, or a phone that was not still when the trip started. Do not act on a number you do not believe.