The regulations limit your airspeed to 250KIAS below 10,000'.
Above 10,000', the regs limit your airspeed to below Mach 1.0 over land (not that you can get that fast in this airplane).Vmo for the Lear45 (I assume you're asking about the Learjet in FS) is 330 KIAS
Mmo and cruise speed is listed at Mach .81.
FYI, all of the V speeds are listed in each aircraft's kneeboard (press F10 from inside the cockpit and click the bottom button the right side of the kneeboard).
Hope this helps!
Ben
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