Yahoo Answers: Answers and Comments for How many miles per hour is Mach .80? [Mathematics]
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From Anonymous
enUS
Sun, 31 Dec 2006 19:16:49 +0000
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Yahoo Answers: Answers and Comments for How many miles per hour is Mach .80? [Mathematics]
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From Andy: The Mach Number is defined as the speed of an ...
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Sun, 31 Dec 2006 20:17:39 +0000
The Mach Number is defined as the speed of an object relative to a fluid medium  for instance air  divided by the speed of sound in that medium. It is a measure of relative speed and is named after the Austrian physicist Ernest Mach
It is often associated with the speed of sound and if an airplane is traveling at Mach 1 it is traveling at the speed of sound relative to the air.
The Mach number is not constant and changes depending on the temperature.
The formula used to calculate the Mach number depends on whether the object is traveling subsonic (M<1) or supersonic (M>1).
In fact at M = 0.8 we are talking about the transonic region. For transonic, compressibility effects and air density varies because of local shock waves and other effects. This is mainly because in this region some of the airflow across the aerodynamic surfaces can be supersonic (M>1).
In fact the equations are complex. To answer the question, I have used a online calculator supplied by NASA  see link below.
So to answer the question we can just take an example. An aircraft traveling at Mach 0.8 at 36,000ft is traveling at 529 mph, and at sea level it would be traveling at 610 mph.
So to give a complete answer to your question you need to supply more information  more specifically the height  as you can see from the answers at differing heights it can vary a lot.

From Tim GNO: The answer to your question is more complicate...
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Sun, 31 Dec 2006 19:25:56 +0000
The answer to your question is more complicated than you probably suspect (see link below). The speed depends on several variables, including the medium involved, the ambient temperature, etc.
Although, if you meant to ask, "What subsonic speed is an aircraft flying at Mach 0.8?"
In that case, simply solve the following equation for 's':
M = s / (speed of sound)
0.8 = s / 760 MPH
s = 608 MPH
(See second link below for specifics of this calculation.)
Hope this helps!

From Anonymous: 608 mph
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Thu, 04 Jan 2007 17:38:56 +0000
608 mph

From David C: Mach 1 is the speed of sound which is 760 mph
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Mon, 01 Jan 2007 12:58:58 +0000
Mach 1 is the speed of sound which is 760 mph
.80 mach would be 760 x .8 = 608 mph
The trick is to know or know how to find the speed of sound and know that the speed of sound equals mach 1.

From sword: Mach Miles Per Hour
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Sat, 10 Dec 2016 20:35:55 +0000
Mach Miles Per Hour

From Lmeister: find the mph of mach one, divide it by ten the...
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Sun, 31 Dec 2006 19:19:46 +0000
find the mph of mach one, divide it by ten then multiply it by 8 and thats the mph of mach .80