I have a SW 26 on my 12T. It has the right 60 tooth drive gear, SW 2-1/2 right angle drive, new cable. I checked the speedo against a speed detection sign set up in my town to get people to slow down on a residential street. When my speedo read 25 mph, the sign showed 35 mph. Slower speeds were closer in difference. Could it be a gummed up speedo head ?
If the head has not been cleaned and lubricated, I would send it to Russ Furstnow and have him check it out.
Drive by the speed detection sign with you modern car to be sure its accurate or check your T with a smart phone speed ap.
I concur with Dan.
I use a GPS in my car which shows the speed. My SW reads the same as the GPS. It had been freshly rebuilt by Russ when I bought it.
If the distance reading is accurate, the speedo head may need adjustment or cleaning. If the distance also under-reads, then the gearing of the drive is wrong.
If you go to the trouble to send it to Russ, you might see if he has a series D which would have been used on your 1912. The series D has the rectangular slots for the odometer instead of the port holes.
Thanks for the info guys