Don't forget to allow for converter slippage. However, what the OP has for RPM vs. MPH would = excessive slippage. I am with you, needs to take it somewhere and have the speedometer certified.
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1972 SS 454 Chevelle LS5/M21/12 bolt 3.73 posi
1969 Malibu COPO L72/M20/12 bolt 3.31 posi
1968 Malibu LQ9/4L80E/12 bolt 3.73 posi
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