Is it possible to recalibrate the speedo?

[Hisvett]

I have a '78 and just added a cell phone mount, now I can see (by using Google Maps) that my speedo is fast by 4 MPH at 40 MPH. I believe back in the day one had to change the magnitisum on the needle drive to get it closer. Not sure if there are any old speedometer shops left now a days, know of any?

 

[brettp]

The honda motorcycles read 10% higher than actual. This had been the case probably since the beginning. My 2007 VTX1800 is the same. In fact, so are Acuras.
You can change it by increasing the tire circumference by the same %. Then your tire speed will match your travel. Or, you can do the math on the fly. If the speedo says 50, you're doing 45.
I know I didn't answer your question. I thought these were all geared, zero magnets involved.

 

[Hisvett]

This might help; https://www.google.com/search?q=mag...wMTYwajFqMTWoAgCwAgA&sourceid=chrome&ie=UTF-8