Hi ulisse,
See post #10 by EDA:
http://www.pprune.org/tech-log/51877...green-dot.html
"I've found the relationship between HOLDING speed and the type of the engines, FCOM DSC 22-20-30 Flight planning / 30-10 Lateral Functions / FMS2 / Holding pattern states:
When no specific speed limit applies, the default hold speed is approximately equal to:
-Green Dot speed on the A318, A319, A320 (CFM) and A321
-Green Dot + 20 kt for altitude lower than 20 000 ft, on the A320 (IAE).
-Green Dot + 5 kt for altitude bigger than 20 000 ft, on the A320 (IAE)."
I guess GD speed is min drag on the Airbus airframe plot, but the IAE engine is more fuel efficient at +20kts for some reason.