In climbout, VNAV PTH acts similar to LVL CHG and pitches for the FMC commanded speed, that speed being defined by a virtual point 180nm after takeoff, based on the track direction at this position, the wind direction and speed as per TOC WIND, FL entered, ISA Dev and CI.
19th Nov 2015 13:04
Nope... VNAV speeds are determined by the restrictions in the legs pages and the climb profile. The pitch is related to power to respect those speeds at the thrust selected. VNAV makes 3 basic calculations based on the initial inputs to the FMC. up to 5000 ( terminal area) 5000-10000 (speed limited) above 15000 when VNAV will transit the thrust to full CLB if a reduction has been inserted at take off. The TOC and ISA deviation is used by the FMC to calculate the OPT/ MAX CRZ and M No. Virtual points 180NM? what happens on a 100 mile flight?
Agreed if you don't put in the forecast winds in the DES FORECAST page or forget engine anti-ice ON/OFF, to allow for increase in N1, the path is not calculated correctly and VNAV will not retain the DES SPD as shown with Msg " drag required" in scratch pad. If VNAV cannot manage the path then VNAV SPD will appear in FMA and possibly " unable next altitude" in scratch pad. A quick look at LEGS route DATA will help you to see the times predicted at each waypoint. Its not rocket science to figure if the aircraft can loose 100 its and 10000 feet in 2mins.. well it is to some;(
CLB PAGE:
The page title displays the type of climb. Normally, the title displays ECON for the economy
climb mode. Fixed speed climbs modify the title.
XXX LIM CLB indicates the limit speed, XXX, is based on leading or trailing edge flaps:
• target speed is 5 knots below trailing edge flap placard speed
• speed is limited to 230 kts if leading edge devices are not completely retracted
ECON indicates the speed is based on a cost index.
MAX RATE indicates the speed is based on the maximum altitude over the shortest period of time.
MAX ANGLE indicates the speed is based on the maximum altitude over the shortest horizontal distance.

Flight Management, Navigation Chapter : 12-11 FMC Takeoff and Climb Section : 41
Fixed climb speeds display XXXKT for a fixed CAS climb speed or M.XXX for a fixed Mach climb speed profile. Reasons for fixed speeds are:
• takeoff/climb acceleration segment constraints
• waypoint speed constraints
• an altitude constraint associated with a speed constraint
• a speed restriction
• a crew entered speed.
Displays ACT when the climb phase is active.