TR-6 Speedometer for OD Tranny

[Patrick Bitton <pbitton@axess.com>]

Someone on Ebay is selling a speedometer that claims it's for OD
transmissions. I asked him the difference and he told me that "Revolutions
are different for the OD compared to the non OD. One is 1000 and the other
is 1120." Is this true? Do I need to replace my speedometer to get a right
reading? It seems that when I feel that I am going 50km/h, the reading is
about 30km/h. Has anyone experienced this while doing this conversion?

Thanks,

Patrick