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That depends upon the speed of sound through the medium in which the speed is being measured.

mach = speed ÷ speed_of_sound_there

⇒ speed = mach x speed_of_sound_there

In dry air at 20oC the speed of sound is [approx] 1236 km/h

⇒ speed_of_mach_0.85 = 0.85 x 1236 km/h

≈ 1051 km/h

At 11,000m (where the air temperature is about -50oC) the speed of sound is approx 86% of that in air at 20oC, that is about 1063 km/h

⇒ speed_of_mach_0.85 ≈ 0.85 x 1063 km/h

≈ 904 km/h

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Q: How many kilometers per hour does 0.85 mach equal?
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