The Stovebolt Site Forums The Shop Area Engine Shop Speedo adaptor on tranny - is it linear or a % adjustment?

I have an adaptor from JC. I put tall tires on my 1954 3100, and went to 16" rims from 15". The speedo was 10 mph slow vs. actual GPS speed. Thus, when speedo said 40, I was actually going 50.

Now here is the strange thing - With the new adaptor, I am spot on at 40 and below, but then I start to have a difference:

40 speedo = 40 actual
50 speedo = 48 actual
55 speedo = 51 actual
60 speedo = 55 actual
70 speedo = 62 actual

I thought the correction would be linear. What am I missing here? Thanks.

Harry