I went ahead and decided that since a new, silkscreened speedometer glass was less than $30, I would try cleaning the one I have and if it got really messed up, I would purchase a replacement.
My speedometer appears to have been opened up once before, as there was no gasket between bezel and the unit.
I started very carefully with just water and saw that although the white silk-screening would look a little fainter when it was wet, it came back as it dried. But, with just water, the glass didn't really get much cleaner. So I checked with Google to see if it thought that soap would remove the ink. It indicated that it probably wouldn't. So I proceeded to clean the glass plate with some Dawn dish soap and my fingers. (Be careful, the edges of the glass are sharp!)
I think that I got a bit overconfident and rubbed some spots more than I should have, as "Temperature" and "Battery" both look a little worse than I had hoped that they would. I would be less aggressive with "scrubbing" the glass (even though I was only using my fingertips) where the lettering is, if I were to do this again.
It seems that a previous owner got some paint or something on the lower part of the glass and tried to remove it was something abrasive. The dark areas you see on the lower part of the glass (other than some residual rust stains) are probably paint I have not been able to get out of a lot of fine scratches on the outer surface of the glass.
Nick 1940 KC Model 1/2 Ton Pickup Project (with '37-'39 Bed and what we think is a '42-'46 Frame) Taos, New Mexico
