speedo question

[64shortbed]

Ok i have a 64 GMC shortbed. Now i have put a 350/350 combo in it. It has the original dana 44 rear with 3:54 gears. I put 255/60/15 on the rear. Now i have searched and searched and figured out what my speedo gears should be. I have a 7 tooth drive gear and a 19 tooth driven gear. Which gives me a 2.71 ratio. What i should have to read right is a 2.75 ratio.

Now my question is my speedo is like 10 mph to high ruffly. Why is this? There is not enough differance between 2.71 and 2.75 to matter that much! So the speedo is original and i have a spare i put in and it sticks (as it sits in the garage right now in says it doing 20 mph) do these speedos not run the 1000 rpm? Is that what my problem is? I cant remember were i read that but it said that they started the 1000 rpm speedos in 1965?
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