Thanks for the replies. The flight manual gives a flaps down, power off calibrated stalling speed of 46 knots. As mentioned before, the Vs0 or bottom of the white arc is 33 KIAS. The landing distance table (short field) is based on a speed of 61 KIAS at 50'.
Comparing the 33 KIAS Vs0 of the 1981 172P to older models (172N's etc) I've noticed that this Vs0 is very low. I assume this could simply be due to a different model of ASI or pitot static system used... any other ideas?
Many flight training books quote short field approach speeds of 1.3 x Vs0 but fail to mention this is based on CAS. With such a difference in Calibrated and indicated Vs0's I believe Cessna should mention this in their manual.
A student may be tempted to reduce the approach speed because of the big margin above the stall on the ASI. For example, 1.3 x 33 KIAS = 42.9 Knots. Using the airspeed calibration table this would give a CAS of around 49 Knots... only 3 knots above the Calibrated Vs0!!! Worth a thought.
(The airspeed calibration table states: 'condition: Power required for level flight or maximum rated RPM dive' - I suppose these speeds may also be different with power off!)