It's a Stweart Warner, most likely one of the tens of thousands or perhaps hundreds of thpusands of aftermarket speedometers they sold from the teens through the 40s rather than a specific make or model.
Hey Mr. Snippy. Looks similar to an old Model A speedometer, but those were oval. My best guess would be something from the late 20's, early 30's.
You can look here to see if you can find a match: https://sites.google.com/site/identifyinginstrumentpanels/home
Google is your friend http://antiquespeedometer.com/inventory.php?item=EF4729A4-2B32-3679-F6B013DB03332FC8
Pretty cool site! I noticed multiple gauges listed at Model 600 (different shapes and faces). The one OP pictured has a trip odometer, which makes me think it was offered as an aftermarket speedo rather than OEM.
That's a pretty early one, likely 1920's. Many different models used the same/similar units with small differences. Are there any case or face identification numbers that you can see?