Just how fast is impulse?

[Timo]

Not necessarily propulsive exhaust, though - what was being tracked in the movie and the TNG episode could have been the sort of exhaust one gets from an internal combustion engine, rather than the sort one gets from a rocket or a turbojet. Such a low-velocity exhaust might not have been all that harmful within Spacedock.

Timo Saloniemi

 

[Crazy Eddie]

Not necessarily propulsive exhaust, though

It is--necessarily--in Relics, since Scotty is able to ascertain from the ion trail that the impulse engine was in full reverse pulling against a tractor beam (meaning the ship was dragged in and did not fly in by itself).

 

[Timo]

But you could also tell from the exhaust of a car whether its engine is on full or idled.

Timo Saloniemi

 

[Crazy Eddie]

Sure you can. What you CAN'T tell is whether the car was in Drive or Reverse. That's a property you could discern from, say, jet engine exhaust or the wake of ship, something directly connected with movement and propulsive force.