Largest Historical Real GDP

BS asked after class the other day, at what point did the U.S. become the largest economy in the world. 

My off-the-top-of-my-head answer was sometime between the end of the Civil War and the start of World War I. I suspected it was even before the Spanish-American War, when informed experts thought Spain would win easily, and in which it had trouble even winning battles much less the whole war.

But that question can be answered factually from the Maddison Project data (discussed in earlier posts like this one). 

In this data, the entities are contemporary countries, so it's necessary to combine them together to estimate the GDP of empires. Also, not every country has data for every year. This is the data for 1880. The U.S. first passed the U.K. in 1879. Bear in mind that this is the UK as currently constituted. It does not inlcude their colonial empire. This was (as in the case with China in the 21st century) a result of greater population: at that time per capita real GDP in the UK was $5,879/yr. while in the U.S. it was $4,866.

Country	Real GDP In Billions of 2011 International Dollars per Year
USA	246
U.K.	204
Germany	175
France	136
Italy	96
Japan	50
Spain	50

Keep in mind that in 2011 International Dollars, the current GDP of the U.S. is about $20,000B/yr.

Here are the figures for 1913, the last year of peace before World War I. The current country is given, followed by the country and its empire in parentheses (for the countries on which we have data).

Country	Real GDP In Billions of 2011 International Dollars per Year
USA	791 (816)
UK	368 (712)
Russia	485
Germany	479
China	343
France	238 (244)
Italy	174
Austria-Hungary	133
Japan	113 (124)
Canada	59
Argentina	51
Australia	45
Mexico	36
Brazil	32

I've added a few countries for curiosity's sake. China was not really a united country at this time, and the large number indicates 4 times as many people as the U.S., but real GDP per capita that was less than a tenth of the U.S. The UK is so large because India was the major economic component of its empire. Towards the bottom I've added Argentina, along with other countries that have now surpassed it.