Impulse drive

Voyager's maximum speed at impulse was stated in the "Timeless" to be 0.8 c, while "Fair Haven" suggests speeds of less than 200 000 km/s. Since "Fair Haven" is almost entire season after the "Timeless", that discrepancy can be explained by possible damage to the impulse drive and maintenance problems due to ship being trapped in Delta Quadrant. However, from several episodes it seems that impulse drive can also propel starships at FTL speeds, although with lower maximum speed and greater power consumption than warp drive (it will also explain how Romulans in "Balance of Terror" had only impulse engines, and yet were able to maintain FTL capacity and interstellar empire), which seems to suggest similar principle to the warp drive. Reference made in TOS about impulse drive using subspace field might support that possibility. Impulse drive also seems to offer great acceleration and manouverability at sublight speeds.

(Incidentally, it also can explain short combat ranges - ship at full impulse can close from maximum to point-blank range in less than a second).

As for acceleration, refit Enterprise in ST:TMP went from Eart to Jupiter in 1.8 hours.

Minimum distance between Earth and Jupiter is 588 million kilometers, according to this page. That gives us average speed of 326 666 667 kilometers per hour. Maximum speed should be (at Jupiter) 653 333 333 kilometers per hour. That gives us acceleration of exactly 362 962 963 km/h^2, or 100 823 045.3 m/s^2.