There has been a lot of discussion on this topic over the years. I took an old rusted 6V horn off a truck in a field and cleaned/refurbishes it for my 58.
Installed as shown below, it draws 11.5A measured by a precision supply at 13.5VDC (high accuracy confidence) and is 96dB measured by an inexpensive SPL meter 6' away at waist height (moderate accuracy confidence).
Obviously some variation will exist from horn to horn, but this seemed like useful reference numbers for some considering doing the same.
