A bit more expanded from my previous post:
The best Rate of Climb occurs at a speed where there is Maximum Excess Power. As jet engines directly produce Thrust, not Power, it is necessary to consider Thrust multiplied by speed (Power = Force X Velocity). Thus, for a given Thrust setting, Power increases as TAS increases. Thrust actually ‘dips’ as speed increases, but then there is significant Ram recovery at higher Mach Numbers, thus further increasing Power at higher speeds. If we examine a Power Required and Power Available graph for a jet aircraft (about the only time that we’re interested in Power for a jet), it is observed that at the higher speeds, Power Available Vs Power Required (thus excess Power) is close to parallel with minimal divergence over a fairly wide speed range. Speed variation of the order of about plus and minus 20 to 30 knots from the optimum, shows only a slight reduction in excess Power with this speed variation.
For a fairly Light jet aircraft such as the A320, a fairly typical best Angle speed is 250 KIAS, best Rate 280 KIAS, and a typical Economy climb speed is 300 KIAS. Best Angle speed is at a somewhat lesser Rate than best Rate speed, so, if the two are compared, the low speed climb will take longer and consume more fuel to Top of Climb. Then, for a climb time of, say, 30 minutes at a difference of 50 KIAS (about 80 KTAS), there will be an incremental cruise of 40 nm required to just achieve the point where best Rate of Climb would have ended. Therefore, in the comparison between climb at best Angle and best Rate, the lower speed will cost you both time and fuel. The best Rate of climb can be refined further to optimize time and fuel. Remembering that up to 20 to 30 Knots ‘Off Optimum’ have only minimal effect upon excess Power, an Econ Climb speed a little above best Rate (+20 KIAS for the A320) will have a negligible effect upon Rate of Climb, but put the aircraft 30 miles or so further down track at Top of Climb. The very slight increase in climb Time and Fuel is more than off-set by the extra distance covered in the ‘fuel expensive’ climb, and elimination of the incremental cruise. If we consider operations for a Maximum Range profile (absolutely minimum fuel), it’s not at all uncommon to see a Climb speed in EXCESS of the initial Cruise speed.