1973 MGB speedo calibration

[skootch13]

I have a 73 MGB with a triumph lt77 5 speed and a chevy 10 bolt rear end, ratio unknown.

I sent my speed to Nisonger for recalibration.  They informed me that they can make the speedo work but the odometer won't work with the speedo.  Something about how the speedo is "1300" and now your car wants to be a "850" speedo.

What are my options?  GPS unit?  Cobble two speedos together to make one?  This is a low mile car so I want to be able to document the mileage and not lose it.

[skootch13]

West valley instruments says they can fix it.  no idea why Nisonger couldn't