Fascinating stuff Cray :) I asked this question because i'm just trying to write/produce something and in it i've got a max/cruising speed of some ASF's and had a 7/11 55 tonner able to cruise along at Mach 1.81 and go to Mach 3.68 when they give it the beans. A 6/9 85-tonner's cruising at Mach 1.14 and Mach 2.38 at full throttle.
Does that sound about the ball park ish for folks?
The Short of it is using Low altitude rules which translates to 180kph per hex, for a 6/9 the max speed is 12 hexes per turn or 2,180kph. High altitude it Ironically is the same for 0-18km of altitude, above that you start getting faster speeds, as under high altitude rules each hex traveled is equivalent to a speed of 1,080kph, as such the ultimate airspeed under the rules with this rule set is 12,960kph (at ~55-72 km).
So for low altitude the following speeds will be achievable
4/6..... 8 hexes or 1,440kph or Mach 1.18 at sea level
5/8..... 10 hexes or 1,800kph or Mach 1.47
6/9..... 12 hexes or 2,160kph or Mach 1.76
7/11... 14 hexes or 2,520kph or Mach 2.06
8/12... 16 hexes or 2,880kph or Mach 2.35
9/14... 18 hexes or 3,240kph or Mach 2.65
10/15. 20 hexes or 3,600kph or Mach 2.94
11/17. 22 hexes or 3,960kph or Mach 3.23
12/18. 24 hexes or 4,320kph or Mach 3.53
13/20. 26 hexes or 4,680kph or Mach 3.82
14/21. 28 hexes or 5,040kph or Mach 4.11
15/23. 30 hexes or 5,400kph or Mach 4.41
A few things to note, as noted the Mach number is for Sea level, at 12 to 20 km the speed of sound is only 1,063kph and not 1,225kph it would be at sea level, as such a Generic "dog fighter" ASF (typically 6/9) would be capable of traveling at Mach 2.03 at thoughs altitudes. Using the High altitude rules all ASFs are capable of 2,160kph at these speeds. Though at 19-20km altitude one could use the high altitude rules and travel at 3,240kph or mach 3.05... An interesting fact, spheroid Dropships at high altitude can travel one hex laterally per turn so yeah you can have an Overlord traveling at supersonic speeds sideways...
If you want something more fluffy from the Novels their are three references (AFAIK) of ASFs traveling at hypersonic speeds (in atmosphere), as low as 500m above the ground, interestingly one of them I believe was a 5/8. So in universe ASFs are a bit faster in an atmosphere than what the game would go with (also in the novels are a few references to bombing at supersonic speeds, which I believe is something they can do in game, and is done in real life).
Endurance is fairly simple, each point of fuel is 36km using the high altitude rules, though only if your using just one fuel point per turn and or cruising with an even number of movement points (I.e. moving is 2, 4 or 6... hexes per turn and not 3 or 5...), as odd numbers cut the distance you travel in by a fair bit (by up to 25%). So the typical 5 ton fuel tank is enough for between 7,200 and 14,400km (on average it would seem closer to the 14,000km figures). Even crazier is the fact that Conventional fighters have twice this range, and if you use the optional rules in Strat ops quadrupled, it's not to hard to get an conventional to be able to fly around the earth once or twice (Meteor HSF with max external fuel supposedly can fly for some 86,000km (unless conventional fighters using external only get 40 points per tank...))!
Using the high altitude rules the speed "limits" are as such
0 to 18 km 2 hexes per turn or 2,160kph
19 to 36 km 3 hexes per turn or 3,240kph
37 to 54 km 6 hexes per turn or 6,480kph
55 to 72 km 9 hexes per turn or 9,720kph
73 to 90 km 12 hexes per turn or 12,960kph
91 to 108 km 15 hexes per turn or 16,200kph
After this your suborbital (in space), unless you speed up to ~27 hexes per turn and enter orbit.