I know generally it is about 170mph. But what is the actual speed in mph that an airliner has to reach before it takeoff's.
It depends on what type of an airliner you are talking about. Even then, take off or rotation speed in every flight is different. Pilots need to consider many variables like air density, aircraft gross weight, lift coefficient (aerodynamics of the plane), and aircraft configuration (flap or slat position, as applicable), according to Wikipedia. This is a few examples from the Aerospaceweb.org,
- Boeing 737 at 100,000 lb, rotation speed is 150 mph / 130 KIAS.
- Airbus A320 at 155,000 lb, rotation speed is 170 mph / 150 KIAS.
- Boeing 747 at 800,000 lb, rotation speed is 180 mph / 155 KIAS.
Each plane has a set of calculations which pilots use to determine a specific speed (V1, VR - rotation, and V2) for each flight. Modern airliners come equipped with their own software and/or computer designed for this purpose.