http://en.wikipedia....German_American
1840–1900. Germans form the largest group of immigrants coming to the U.S., outnumbering the Irish and English.
First, for those not familiar with America there are some warning signs in this data.
America was officially founded in 1776, way before 1840. I still have seen no evidence that Germans were a majority or even close to it in the American colonies outside of Pennsylvania, 1/13 colonies that made up the first states.
Another thing to determine is how do they consider who is German? Are they doing it by last name? Are they doing it by survey?
I was born in Western Ohio and there were a lot of Germans who settled there but most white people I know from there do not say "I'm German"...it is usually "oh I'm part Irish, German, English, blah blah Shawnee Indian, blah blah"
How do they categorize those people. Some people will just say "I'm a mutt" or "I'm American". Sometimes people will say "my great great grandfather is German" but then everyone else in their family was not.
White Americans, the vast majority outside of some isolated Northeastern urban ethnic ghettos or some very rural towns highly ethnically mixed at least with two European nationalities, most more than that.
I have met "pure Italian" or "pure Irish people" in Chicago when I lived there, but they were usually only 3rd or 2nd generation. I have never met that type of "pure nationality" in other areas of America that are not prime immigration areas because people have never built up a ethnic community that makes it easier to meet and marry someone of the same ethnic group. Plus I have never met a German American (or a person with a German last name) who was not Amish from Pennsylvania or Ohio who spoke any German at all. The point in that is the ethnic barrier to meeting and marrying someone is very low, at least within one's race in the United States if both parties speak English as a first language.
My thinking about the statistic in Wiki (if even correct) is that the people who consider themselves German (or uni-ethnic) is higher than the average white American who does not identify with any one ethnic group and therefore can't fit into the sample.