Hi,

I've got a question regarding the Airbus A320. When planning a flight in the main menu and selecting a VFR flight plan with more than 10,000ft in height, the navlog indicates that the selected aircraft (a320 neo) has a cruise speed of somewhere around 450kts. 
 

Now that I’m flying, the aircraft indicates that speeds above 360kts are overspeed. Am I doing something wrong? 
 

I’d love to hear your opinion. 

The speed tape shows indicated air speed, measured through the plane’s pitot tubes, which is dependent on air pressure.  At higher altitudes pressure is lower which means that indicated airspeed shows much lower than the actual speed that the aircraft is traveling over the ground.  In the 320 if you look at your displays you will see a small number with the letters GS beside it, which is your ground speed measured by gps and or ins nav systems.  That is your “real” speed.

450 knots is your airspeed over the ground; High in the air you're moving 350 knots.

If it's hard to understand, visualize the individual atoms of air. On the ground, they're close together. Up high, they're far apart. If your plane wants to hit 12 septillion particles each hour, it must move faster up high where the particles are further apart to do so. The number of particles your plane hits per hour is, arguably, called airspeed.

Thanks for the explanation, @ShawnG and @WestAir. I’m quite new to this. I’ve found the ground speed. 

As others mentioned, you are seeing the difference between Indicated Airspeed (pressure in the pitot tube) versus actual speed over the ground. 

360 KIAS (Knots Indicated Airspeed) would be very fast for a 320 and not very efficient. I fly a 320 for a major carrier and am killing time while waiting for the GB download for MSFS.

The Flight Management Computer has a Cost Index which tells the box to prioritize time (speed) at the expense of fuel burn. At a typical CI of 45 you would see climb speeds from 290 to the 320's based on weight, winds aloft and cruise altitude. As you reach the 33,000 to 37,000 feet where the 320 is happiest you will see your low speed cues "the hook" of VLS, Alpha Prot and Alpha Max come towards you as the Max Operating Mach comes down the airspeed indicator. The Green Dot is lift over drag max, which is optimum for climb rate and fuel efficiency (although it is slow in cruise - Only American Airlines pilots fly that speed during contract negotiations).

I drop 300 to 320 in the Descent Page during Init to keep up with traffic in the descent going into busy airports. The default at a CI of 45 is too slow for the real world.

I have no idea what FMGC functionality exists in MSFS.