Ugh, this is… complicated. I use a visual editor to set them as a function of rarity, upgrade level, or both.
These are the current non-massive values (there is a -15% penalty on ALL of them on massive environments). Non-massive values are only relevant to bombers (because fighters have infinite speed), but the values are nevertheless present in the code for all spacecraft.
Units are “native pixels per second”. Each rarity gives +30.
H&C 101/201/301
1000.000000
937.500000
875.000000
M-400/404/408
875.000000
812.500000
750.000000
BX-7/8/9
550.000000
500.000000
450.000000
Engines are linearly added on top of the spacecraft speed (before any -15% massive penalty). They are even more complicated, because they use a combinatorial matrix of rarity and upgrade levels. Upgrade level is on the X-axis, rarity is on the Y-axis.
Example for first chart: A common non-upgraded engine has no bonus. A legendary non-upgraded engine has up to +15. A common fully-upgraded engine is +20, and a legendary fully-upgraded engine is +40
