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Speedometer gear

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So I just bought another 70 SS454 which had the rear gears changed to 3:55s.  While I like this gear a lot for local cruising (and tire spinning of course) it is not so good on the freeway.  That said, I think I will leave it as is at least for now.  Can anyone tell me if I can simply change the speedo gear in the transmission to correct the speedometer...it is currently reading about 90 MPH when doing 70 on the freeway.

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Here is another Calculator Tool to help you .  If you know what your Drive & Driven gears in Transmission are, it will help in deciding what course of action to take.  I had the same issue when I replaced the Rear End & gears to 331:1 ratio 12 Bolt Posi from a  273:1 ratio 10 Bolt if I remember right.  I ended up putting a Gear Reduction devise between speedo cable at Tranny connected to devise and to Transmission.  Damned if I can remember what the Technical name is.  I was way off and brought actual speed within 1% of accuracy. 

Pretty sure you will get a lot of help on this.  :)     






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