I am trying to find a comparison of the german and american grading
scales to list on my resume in Germany.
 
What I am finding impossible to find is a reliable scale (with a
citable source) that will allow me to "translate" my american grades
to the german scale. Namely, that my 4.0 from high school will be
looked upon with deep skepticism if translated to a 1,0 because that
has been explained to me as "impossible" from german friends.

My major goal is to be able to write on my resume something like the
following:
My High School, final GPA 4.0 (equivalent to x,x on the German grading
scale)
My University, final GPA 3.60 (equivalent to x,xx on the German
grading scale)

Hello Rheinland,
While your German friends say it is impossible, I say it is not. But a
100% accuracy in comparison between the two grading systems cannot be
expected. For sure, the 1 to 5 and 4 to 0 systems are different and
exact equivalents are hard to find. But I will give a try on comparing
the German and American grading systems.

Explanation of the German examination types and grading system
http://www.uni-augsburg.de/english/programs/explanation.shtml

1.0 or 1.3  	 	1.00 - 1.50 	1.00 - 1.50 	very good
1.7 or 2.0 or 2.3 	1.51 - 2.50 	1.51 - 2.50 	good
2.7 or 3.0 or 3.3 	2.51 - 3.50 	2.51 - 3.50 	satisfactory
3.7 or 4.0 		3.51 - 4.00 	3.51 - 4.50 	adequate
below 4.0 		below 4.0 	below 4.5 	fail

Here is how it is compared to the letter grading system in the same
web page:
1.00 - 1.50 = A 	
1.51 - 2.00 = A- 	
2.01 - 2.50 = B+ 
2.51 - 3.00 = B
3.01 - 3.50 = B-
3.51 - 4.00 = C
4.01 - 4.50 = D (only Staatsexamen)

Here is the basic American model:
Fulbright Commission Argentina - The American Grading System
http://www.fulbright.edu.ar/html/english/eic/us_gra_sys.html

GRADE 		MEANING 	QUALITY POINTS		
A		Superior	4
B		Above Average	3
C		Average		2
D		Below Average	1
F		Failing		0


So if we were to compare the grading system, I came out with this
calculation:

AMERICAN 	GERMAN		PROBABLE COMMENT
4		1		Excellent
3.5		1.5		Very good
3		2		Good
2.5 		2.5		Satisfactory
2		3		Average
1.5		3.5		Below Average
1		4		Poor
.5-0		4.5-0		Fail

The above however would be only an approximation, and should not be
used as an official conversion standard of grades across systems.

Let me use a local example. I studied college at the Ateneo de Manila
University here in Quezon City, Philippines, and we used the following
grading system, based on the American model (I took this from my
transcript of records). There is a comparison to the
1-highest/5-lowest system:

GRADE 	COMMENT		QPI	APPROX EQUIV IN 1-5 SYSTEM
A	Excellent 	4	1
B+	Very Good	3.5 	1.5
B	Good 		3	2
C+	Satisfactory 	2.5 	2.5
C 	Sufficient 	2 	3
D	Passing 	1 	4 or Incomplete
F	Failure 	0 	5

Compare this to a nearby school, the University of the Philippines
Diliman. It used the 1-highest/5-lowest grading system, but I've even
heard of grades of 1.25, 2.75, 2.25, and not just 1, 2.5 or 2, and you
can see how confusing this gets when you try to translate it into the
American system. Thus I can say that the attempt of the person who
copywrote my school's transcript document was also an approximation,
and can't be made into an official basis. If I were to transfer
school, for example, my grades would certainly be interpreted by the
receiving school personnel on their own discretion, regardless of the
guides given by the former school.


Note that the use of letter grades can differ between schools. See in
my grading system, there were no A-s or B-s in the official grades.
But there are schools who use such grading systems:

American University Washington grading system
http://www.american.edu/american/registrar/transcripts/grading.htm
Grade  		Quality Points (QP) 	4-point  	3-point *
A (Excellent) 				4.0 		3
A- 					3.7 	 
B+ 					3.3 	 
B (Good) 				3.0 		2
B - 					2.7 	 
C+ 					2.3 	 
C (Satisfactory) 			2.0 		1
C- 					1.7 	 
D (Poor) 				1.0 		0
F (Failure) 				0 		-1
X (administrative penalty) 		0 	

Thus, you can see, trying to find an exact equivalent of grades in
other schools presents uniquee problems, simply because schools are
free to use their own grading systems. Much as I might like to have a
'universal' grading system, I have heard of no such effort, and
schools in certain countries use the grading system they please.

However, in your case, if your 4 is the highest in your grading
system, then I believe it compares with the German 1-highest. I'm sure
though that when you get to the lower grades, there'll be a problem in
conversion. Perhaps you could approach your school's registrar, or
someone else you know who works in the same capacity, and ask his or
her opinion on the matter.

For interest's sake, here is another unique grading system from
another American high school, though it somewhat adheres to the usual
model:

Overland High School grading system
http://blazernet.ccsd.k12.co.us/GeneralInformation/grading_system.htm
A-Excellent
B-Above Average
C-Average
D-Below Average
E-Passing with credit, indicates effort rather than attainment
F-No Credit
S-Pass or Pass/Fail
U-Fail or Pass/Fail
I-Incomplete
AU-Audit, no credit
WP-Withdraw Pass
WF-Withdraw Fail


Google Search terms used:
germany us grading system
german american grading system
american high school grading system

I hope this has been a most helpful answer. If you need anything else,
or have a problem with the answer, do please post a Request for
Clarification and I shall respond as soon as I can. Thank you.

Dear Rheinland, 

Basically Techtor is right, but I have yet another suggestion for you
- instead of "translating" the grades. You could write - if we're
refering to CV or a similar description of yourself - the percentage
you were ("the highest 87%"), or in the case of a university degree,
refer to the known Latin terms:
summa cum laude, cum laude, magna cum laude. 

You could see an example at the University of Connecticut site (please
note that this changes from university to university and from one
department to another):
http://www.ucc.uconn.edu/~regsdh06/gradhono.html