Yes, maybe you had a mod on?

How your speed hit that fast when the counter cannot go above 999 is beyond me though

@Chips
Yes, it does go above 999, but not to what he is talking about. I've modded my game, went into cruise, then cut my engines. I then turned around and was flying backwards. The speedometer was up at 2500!

@Robbio2000
How did you know that you were near that speed? You would have been on and off the nav map in like, ha ha, HALF A SECOND! (sorry for caps)

I had a similar experience recently, but I think I can explain it:
a) Installed TNG 2.7.6 but because I wanted to explore a bit (SP) I run Open SP as well and edited a save game with Ioncross Char Ed.
b) Several times (and unexpectedly) I got a warning from ICCE that I had not mounted an engine. By default it loaded a Lib-Dreadnought engine, so i changed that to "Light Engine" (for a Defender btw). I played around with differrent settings - and I had to change the engine several times. But ICCE seems to keep all engines.
c) When I got off at Gas Miner Naha and speeded up I had a ridiculous speed (crossing a map in a few seconds - if I didn't hit a station or a star )
d) first I thought this might be a 'feature' of this TNG-version; but when I tried to get another ship (and only there) I saw that my ship had five or six Light Engines working.

Conclusions:
1. After deactivating/selling the Engines (apart from one of course) the speed was normal again
2. The 'uber-speed' was definitely not only an addition from top speed of the several engines (a+b+c...=v).

I don't know if the game multiplies the speeds - but my game experience with the 'uber-speed Defender' was a little bit like the WOW many years ago when the set for lightspeed in "Star Wars".
Awesome, then BOOOOM.
In FL the systems are overcrowded for VERY fast driving. It's like full throttle in a Porsche Turbo during rush hour in Tokyo