Here’s a actually reasonable explanation for why ships in space sims (eg. Freelancer) don’t use fuel, don’t follow the laws of physics and have arbitrary speed limits. LEARN, you shitty sci fi writers!
The tech behind regional propulsion methods works by locking on to the regional gravity source with modified tractor beam technology for an engine and pushing/pulling the ship against the gravity source with energy (thus not requiring any reaction mass), the higher your speed the higher the instability of the tractor engine and any speed higher than the gravity source’s speed tolerance index would result in loss of useful acceleration, conversely traction drag increases with speed so braking force is also increased with speed, to a point of becoming dangerous.
This is also the reason larger ships have lower speed limits – because the force exerted by their engines from the gravity source is much, much greater, thus the speed tolerance index for large ships is much lower and the linkage becomes unstable much earlier on.
Non-warp cruising to high speed uses a more traditional propulsion technology to accelerate ships to high speeds unobtainable by regional propulsion technology, however obviously such high speeds require reaction mass to attain, and require reaction mass to safely decelerate due to the aforementioned traction drag that can make turning on your regional engine the same as making your ship a bullet lasso’ed to a brick wall.
There are two types of reaction mass thrusters: cruise thrusters and boost thrusters. Cruise engines are highly efficient engines that provide slow but consistent acceleration consuming minimal amounts of fuel good for long range and exploration applications, boost thrusters have a much higher thrust output albeit at a poor efficiency and is only intended for a temporary speed boost in combat.