Overspeed at 400 knot air speed.
Cruise at 1100 knots ground speed.
Learn the difference

The fact is, when you fly at 300 knots IAS (indicated air speed ?) at 5.000 feet, you're much slower than when you fly at 300 knots IAS at 40.000 feet.

Air is less dense at high altitude, so your plane flies faster there, even if the airspeed doesn't seem that high on the gauges.

Adding to the excellent responses above, the following two statements in the aircraft.cfg control overspeeds: (example from the B777)

max_mach                = 0.90   // FL180 and above
max_indicated_speed = 330    // Below FL180   //Red line (KIAS)

Below 18000', you must not exceed 330 KIAS.
Above 18000', you  must not exceed .90

Simple but, you still must respect the 330 KIAS as you climb above FL180 to keep the aircraft from being destroyed by heat and potential turbulence.  You will notice that KIAS readout drops rapidly at higher altitudes so you can continue to throttle up as it declines.

You can change max_mach=2.02 as desired upward to eliminate the overspeed warning.

Thanks for everyone's help but I have managed to partially solve the problem by changing theairspeed indicator to get a digital readout for more accuracy the overspeed warning came on at 369kts at any altitude which is obviously wrong. I then substituted the cv990A air file as a test after trying many other things. I was then able to fly it after finally getting it in the air to its recommended cruise M2.02 with no overspeed. That is as far as my limited knowledge could take me. There must be a setting in the air file that is wrong but what do you change?Can someone help me on this

Did you even read the link I sent you?

Why did I even take my time to dig up an old post of mine to help someone out if they aren't going to read it.

Lol, I'm having visions of a Concorde streaking through the sky, and has it's wings begin to tear off,  the pilot turns to his co-pilot and says, "This will never do.  Only 425 knots. We'll never get to Paris at this speed...."  

Wow I did not intend to step into a hornets nest. Having never tried the concorde before I was in a learning curve. Yes I understood  what everyone explained to me. what got my curiosity up was why a lot of freeware a/c do not act in the same way. eg what  triggers the over speed warning. Also in case I had a bad down load I re-downloaded the Concx and made no changes except for the engine fix.It worked fine until reaching around 35000 ft when the ac started porpoising and the higher you went the worse it got, reducing the vertical speed from the default 1100ft made no difference nor did varying the air speed or the mach number. Am I still doing something wrong? There was no over speed or stall warning showing and we were still a long way from 60000 ft. Should I have started reducing my vertical speed sooner ?