I think we'll have to agree to disagree: I think Art Deco has more in common with the "Gleaming Brass" variety of Steampunk than with the Diesel aesthetic. (I'm obviously ignoring sartorial fashions.)
Basically, Art Deco demands simplicity and elegance in all elements of design, from the most structurally-critical members to the ornamental shapes of the doorknobs. I think of the Diesel aesthetic as saying "screw elegance, and screw ornaments while you're at it. Does it work? Will it keep working? Will it be easy to fix?". The Loveless Spider-tanks are definitely Diesel Punk, despite possibly being steam-powered (in which case they're both Diesel Punk and Steampunk).
Basically, Art Deco demands simplicity and elegance in all elements of design, from the most structurally-critical members to the ornamental shapes of the doorknobs. I think of the Diesel aesthetic as saying "screw elegance, and screw ornaments while you're at it. Does it work? Will it keep working? Will it be easy to fix?". The Loveless Spider-tanks are definitely Diesel Punk, despite possibly being steam-powered (in which case they're both Diesel Punk and Steampunk).