I have found no US powered road race before this one in Wisconsin. Researching steam road vehicles up to 1875 in the US, there does not seem to be an occasion when two such vehicles could get together in one place.

The first staged steam race is easier to answer with the 3 August 1808 race in London where Trevithick he pitted his "Catch-Me-Who-Can" against any and all comers, horses or whatever, for a 24 hour run. The engine was guided on a wooden boardwalk to avoid getting stuck in the mud. Much money changed hands.

Still in the UK, steam drags, omnibuses and such were around in the late 1820s with Gurney, Hancock, Maceroni, etc and I have scoured the papers to find evidence of a staged or informal race with no luck.

Eventually we come to the record of a prearranged match race of two self-powered road vehicles over a prescribed route at 4:30 A.M. on August 30, 1867, between Ashton-under-Lyne and Old Trafford, a distance of eight miles. It was won by the carriage of Isaac Watt Boulton, one of six he said he had run over the years, perhaps driven by his 22 year old son Philip. The race was against Daniel Adamson's carriage, likely the one made for Mr. Schmidt and perhaps driven by Schmidt. The reports do not indicate who was driving, perhaps because it was illegal--the red flag law was fully in force. Boulton's carriage had been converted from a scrapped John Bridge Adams light-rail vehicle. This is recorded in the contemporary press and in several books. I have IWB's own notes on the matter in front of me at my desk, and wish he were rather more wordy. So, two "motors" racing appear to make the First Motor Race anywhere.

In vogue? I guess steam carriages were in the air for a few hundred years and frequently built from 1800 on. Technical and legal roadblocks continually kept them from overtaking the horse until cheap liquid fuel became available.