Most commercial aircraft can't climb to their maximum cruising altitude when fully loaded, they have to burn off fuel before reaching a final cruise altitude by step climbing (leveling off at a lower altitude for a short time). This is not a problem because ATC will normally level an aircraft at intermediate altitudes during a normal flight for air traffic reasons. Commercial aircraft also avoid climbing utilizing VS. Normally aircraft climb using a form of airspeed hold that allows the VS to vary as the aircraft climbs. Large jet engines, as do almost all engines, lose power as the aircraft climbs higher. Most large commercial aircraft can't maintain 1800 FPM all the way to the maximum altitude of the aircraft, they may only be climbing at a few hundred FPM when they reach maximum altitude. If they're very heavy, they won't even reach their maximum altitude.
Flight simulator (default aircraft) don't have a way to climb at a specific airspeed. As you climb, reduce your VS so that you maintain the correct airspeed in the climb. You can also just set 500 FPM, that should get you to most altitudes without problems.
Below 10,000 ft-----------------250 KIAS
10,000 ft. - FL280---------------290 to 310*
FL280- Final cruise alt.----Mach .70 to .75*
*Varies with aircraft type and load. Aircraft manufactures determine exact speeds, these are approximations.
Jets are fastest around FL280-FL 290. Above that they travel slower but burn less fuel. At high altitude, aircraft fly in reference to the speed of sound(Mach 1.0). The speed of sound varies with temperature, traveling slower in colder air. It's colder at FL400 than at FL300. An aircraft that is flying at Mach .80 would be moving thru the air faster at FL300 than FL400. Fuel burn would be higher at the lower altitude. A good efficient altitude that balances speed and fuel is about FL330-FL340. On long flights you will want to fly higher to save fuel and increase range. Winds aloft also play a part in altitude selection.