I think it has more to do with the government/military original
specifications, I would think it goes something like this..

Military "We'd like a M2.5 aircraft..."

Manufacturer "Ah but they would require a variable inlet more
development work and thats more expensive!!"

Military "so how fast can you go without all the extra expence?"

Manufacturer " about M2.0"

Military " Ok close enough"

These figures are then carried through the life of the program, even
when those figures are exceeded by a large margin..


cheers


I've seen it go both ways. I've seen many say a clean F-4 could no
way in hell break 2.2 clean despite the fact it reached 2.62 when it
was going for the speed record (yes I'm aware of the water injection
etc. etc.) On the other hand there was someone a while back that said
they were familiar with an individual who reached 2.83 in an F-111F
briefly even though it's generally listed as 2.5. I know I remember
reading that it was limited to five minutes at a shot over 2.2 or so
because of heating. I guess the only way to know for sure would be to
get a clean aircraft up to it's optimum altitude, top of the tanks,
and put the pedal to the metal until you either stopped accelerating,
were about to exceed heating limits, or were out of gas. LOL I wish
they'd do that for aircraft about to be retired anyway. I'd have
loved it if the Blackbird would have went out with new high marks for
speed and altitude. Oh well.