As far as I remember, the 880 was cruising by design at Mach .88, and I heard the 990 was even faster, possibly Mach .90...
Compare this with the 707, 727 and DC8s of the same era, we were cruising the 707 and 727 at Mach .82, and the DC8 at Mach .80... After the oil crisis of October 1973, these cruising Mach numbers were reduced to .80 and .78 respectively, I assume they reduced the numbers for the 880/990s as well.
Nowadays, the 747-200/300 cruise at Mach .85 in their early (heavy weight) cruise portion, reducing it to .84 and even .83 as they near the end of their flights. I understand that DC-10 or MD
-11 are generally .82 to .83, and the 767 are slower, around .80 or .78... Maybe Rick767 can clarify...