That's going in my formula book, thanks...

Lake Effect,

Good to see you made it on here!

Scott + Mary

I could design and build a fixture to check speedometers. It'd be real easy.
But would it be worth it? There are so many variables that could throw it off after calibration.
That's why they build GPS' and GPS based speedometers.

If you put 100 psi into a 60 mph speedo, it will blow a permanent bulge into the brass Bourdon tube. It will then be trash.

I've been inside many speedos. There is not a calibration provision on any other than Airguides. All the rest you just gotta carefully bend the attachment tab for the rear edge of the Bourdon tube. Careful is the watchword here. And true calibration? Not really - you CAN adjust some degree of fastness or slowness out of it.

You usually have to break the glue seal between the case and the bezel around the glass. Easy does it or you'll crack the bezel.

Good luck, and don't forget that speedos are cheap so you can buyanother if you break this one,.