cosmos Kramer wrote -
Coincidentally I had exactly this scenario last night, so I can back up with some numbers too. Cruising at 360 top of clouds with continuous light turbulence (what other usually report as moderate, for some reason). Max 381 with a 9 knots margin between low and high speed buffet (240 to 249, with 249 equal to approx .79). We climbed at it was smooth at FL380.
...
After 15-20 mins the margin increased to about 15 knots (still flying close to the upper band giving me 13-14 knots to low speed band).
In 15-20 minutes your margin increased by 6 kts?
Looked at 737NG buffet charts. 20,000 lbs change(9000 kg) changes buffet speed by 18 kts. That's a lot longer than 15-20 minutes.