l33t n00b here. I can’t find this answer online unless I am just crap at googling, but I understand the whole cockpit HUD now, except the bar to the immediate right of the speed/throttle bar. A thin line that accompanies it. Is it your max speed?
I think what you describe is the current throttle range, which will change depending upon how many pips you have to engines. Max speed will go beyond that if you boost.