Mach number as weight changes

[XTurb]

Why does Mach number seem to decrease as weight reduces during a flight (i.e. fuel burn)?
Should it? Or is it just my imagination?

[EDML]

Because the speed for optimum range or a given cost factor will decrease
with decreasing weight.

[XTurb]

Cheers for that. Yeah, that makes sense. I need to go back over my ATPL notes...

PWA!

(PPRuNe wins again).