Speed can build up very quickly on heavy jets in dive.
Here is an example of my recent F-84G's Weatronic data collection.
This is taken from the GPS readout that uses a SIRFIII unit.
This unit is known for being very fast in data acquisition and quite precise.
The pass was done in head winds ( quite calm however ).



I have extensively modified this aircraft and I strictly discourage any owner of a stock F-84G to try to reach these speeds.
The tail and ailerons would flutter and destroy the plane.

The maximum speed test was done from a high altitude in dive with 50% of power up to the runway.
This was done as part of the flight test program I always carry out for every plane.
It is only intended to demonstrate the flight envelope, strength of the airframe and validate the modifications. Such a speed is totally unscale and not interesting for this kind of aircraft.

So you see how fast can an old draggy airplane go.
Imagine what a slick modern airframe fitted with a decent engine can do.
It requires serious building skills and some airmanship to be safely operated.