Well,
ex-President Grant could have ridden the car. It was common to refer to him as "President" Grant.
I've been reading the book
Dark Horse about the nomination, election, and presidency of Garfield. During the summer of 1880 Grant was in Colorado vacationing while some Republicans were maneuvering to get him nominated and elected for a third time. The delegate vote was split and after many, many votes Garfield was nominated.
Brian Norden