MickeyP, There should be some code letters/numbers stamped into the 12 bolt axle passenger side axle tube. Those will help identify what the differential ratio is in the axle as well as what the axle originally came from. Usually a 3-letter code is what is used to ID the ratio and the other data is for original application and where/when manufactured. See what you can find and post here so we can try and provide further help. FYI - from your rotation count, you most likely have a ratio in the 2.56-2.73 to 1 range. That coupled with your 1957 transmission 1:1 output would likely cause the misread at the speedometer as others mentioned before the original rear ratio would have been 3.90 or 4.11 to 1. More info when we hear back on your axle details.
