How about this idea:
Cruise speeds
Light Fighters 370 m/s
Heavy Fighters/freighters 360 m/s
VHF/Transports 350 m/s
GB's/bombers (liners?) 340 m/s
Cruisers 330 m/s
BS 310 m/s
VHF are heavier, but have about the same engine as LF/HF and bombers are even heavier, while freighters are heavier as they have bigger engines on em as well. Same story for transports (you earn more if you are faster at your goal) GB/Cruisers/BS are just heavy (maybe make a clear difference for GS and make them cruise at 350)