There isn’t much fuel onboard so below I’ve attached a brief procedure for how to guarantee 5,000 Mph (TAS). I have hit 6.5k Mph but only after a glitch kicked in so unfortunately I don’t know how to replicate it.

Procedure:

1. Start from USS Beast’s catapult.
2. Deactivate activation group [8].
3. Throttle to 25% with elevator pulled all the way back.
4. Press catapult launch button.
5. At end of runway, press activation group [1] to separate from launch trolley.
6. Ascend to 35,000 feet at 70 degrees of climb—don’t begin leveling off before then.
7. @35k Ft, re-activate gyro with activation group [8].
   
   
   
   • use light amount of trim to assist leveling process.
   
   
   • plane should level-off at 65-70k Ft.
   
   
8. Once at 65-70k feet, use trim as needed to keep plane at descent of 10 degrees (max trim should do this).
9. After applying trim, throttle up to 100% power.
10. Once around 3,500 mph use trim to keep the nose downward by 5 degrees. This should bring you 5.3 to 5.5k Mph (TAS).
11. Enjoy!

Notes:
1. The flat-nosed peg in the cockpit is a quirky trick to lower the aerodynamic drag. It brings the plane’s drag (not including launch trolley) from 207 down to just 115 Drag Points.
2. This plane is absolutely a one-trick pony.
3. It has 21 Blasto J50 engines brought into the fuselage which is why fuel-burn is so aggressive.
4. Enjoy!