The earliest use of "for Economical Transportation" that I found was in a 1925 ad in a Floyd Clymer's Scrapbook. Following my own usual advice, I Googled "for Economical Transportation" and got dozens of hits, mostly for the repro signs, but a few that had a little info. I got different hits when I included "Chevrolet" and when I used "Economic". There are little bits of info out there, one seemed to indicate the slogan was in use in 1922. I'll check some more Google sites and also dig deeper into my library.