Does anyone know what make and model of American car comes with the loudest standard horn?
There is absolutely no compilation of that kind of data, nor is that the type of thing anyone evaluates. However, it’s super easy and not very expensive to retrofit any car with an aftermarket air horn from the parts store that will make it sound like a Peterbuilt.
Not sure if it is the loudest horn, but a “low-note” horn from any large GM car(such as the 2005 Buick Park Avenue/Cadillac DTS) is PLENTY loud. AC Delco part number D1922C. “F” tone at 385 Hz frequency. I just installed one on my 1996 Acura Integra and am no longer embarrassed to use the horn. It could be purchased from any GM dealer using part number 12368064, list price is $59.83 but could be purchased for much cheaper than that online.