Ratio is elapsed seconds divided by AToF time.
Release VS2013 on HP xw8600 Xeon E5440 2.83Ghz

Test	Seconds	Ratio	Seconds	Ratio
	VS2013/x86		VS2013/x64
AToF	0.156	1.00	0.140	1.00
atof	0.249	1.60	0.234	1.67
strtod	0.250	1.60	0.234	1.67
scanf	0.468	3.00	0.406	2.90
strStream	5.990	38.40	4.789	34.21
stringStream	6.085	39.01	4.930	35.21

(fewer seconds is faster, each group in order fastest to slowwest)

AToF is a hand written version of the standard atof(). I found the code on the web at:

I am assuming the default atof() and strtod() are slower because they deal with language/locality issues.
StringStream is the slowest way to convert a string back to its binary form.

WARNING - atof() and strtod() internally call strlen() on the input string. If your input string is a pointer to a region inside a very large text document, it will take a long time to calculate the string length. I had a case where I was extracting floating point values from a file I memory mapped. To improve performance, I had to zero terminate each floating point character sequence. Since I did not want to alter the memory mapped file, I copied the text to a local buffer.

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I created a similar test in the reverse direction and here are the results:
Ratio is elapsed seconds divided by fcvt time.
Release VS2013 on HP xw8600 Xeon E5440 2.83Ghz

Test	Seconds	Ratio	Seconds	Ratio
	VS2013/x86		VS2013/x64
fcvt	0.608	1.00	0.546	1.00
sprintf	0.858	1.41	0.764	1.40
gcvt	1.217	2.00	1.092	2.00
strStream	2.886	4.75	2.091	3.83
stringStream	2.933	4.82	2.028	3.71

(fewer seconds is faster, each group in order fastest to slowwest)

Conclusion: fcvt is fastest