[idlewords]

If you want to go by top speeds, for the 787 (entered service 2011) it's Mach 0.9, for the A-350 (2015) it's 0.89, while for the 707-120 (1958) it's Mach 0.91. The 747 (1970) can go Mach 0.94.

Modern jets are built to fly slightly slower than early jetliners, both in normal operation and at top speed. The reason is fuel economy, but the difference is real.

[WalterBright]

That doesn't disagree with what I wrote.

[idlewords]

You're saying planes have a top speed faster than their cruise speed; I'm saying it doesn't matter which of the two metrics you compare on, older jets are faster on both.

[TylerE]

Mach really isn't an appropriate unit here as it is highly depended on altitude and atmospheric conditions.

[AnarchismIsCool]

No, try again. Not correct in any way.

[TylerE]

Mach is highly depended on altitude and atmospheric conditions. It is a ratio, not a speed.

[idlewords]

A specific Mach number is what airliner airframes are designed to fly at; it's absolutely the correct unit to talk about in this context. If airplane A is designed to cruise at 0.94 mach, it is faster than airplane B that cruises at 0.89 mach.

I don't understand the need for all the smoke and din in this argument thread. Old passenger jets flew a little faster than modern ones and that's okay!

[TylerE]

Mach 0.94 at 40,000ft is slower than Mach 0.89 at sea level.

That’s my point. It. Is. Not. A. Measurement. Of. Speed.

[AnarchismIsCool]

Still no...

It's dependent on the speed of sound, which is only dependent on temperature, not specifically altitude contrary to popular belief.

[jjk166]

Temperature varies by altitude. Mach 1 at 30,000 ft is 677mph, mach 1 at 35,000 ft is 663 mph.

[AnarchismIsCool]

Per ISA yes