Is it the speedo gear?

[68 ReNay]

Hey Guys,
i posted a long time ago about a bouncing speedo needle problem. you all offered several suggestions - thank you - unfortunately, nothing i tried fixed the problem. it still bounces from zero and tapers to steady by 42mph.

a couple months ago i discovered my speedometer is off. it's a new OEM speedo, but most likely the original one i replaced was off too, since my tires are larger than the original diameter tires.

but my question is this, should the speed be off a consistent number of MPH's, regardless of speed, or does the difference increase with speed?

i checked mine out and here's what i discovered:
(left side number is actual MPH, right side number is my speedo reading)
30mph - 30mph
35mph - 37mph
40mph - 43mph
45mph - 49mph
50mph - 55mph
55mph - 62mph
60mph - 67mph
65mph - 73mph
if that doesn't sound right, what else should i be looking for?
if that seems reasonable, how do i decide what number of teeth in the gear i need to buy? i know each added tooth adds an mph or two, etc. but do i 'gear' it for what speed i drive most often (ie. highway)?

Please help!
P.S. it has a TH350, if that matters.