Rear end change and speedometer effect

Old May 6th, 2019, 05:30 AM
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Rear end change and speedometer effect

I have a 69 Cutlass that a previous owner did a lot of changes to. He put in a 455 and judging by the high rpms a=it seem he swapped out the rear end as well. At 60 mph my speedometer reads 75-80. It is a real gas guzzler and I am not concerned about setting track times. I haven't had a chance to pull the rear end cover and count teeth to see what I have. I may well put it back to original but for the meantime would like to have a accurate speedometer. Can I change the speedometer gear to correct this issue?

thanks,
Steve
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Old May 6th, 2019, 06:06 AM
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Yes Steve you sure can. You'll likely be changing both the drive gear and driven gear, depending on which housing you currently have. If you're lucky, you'll only be swapping the driven gear but you have pretty big spread of inaccuracy. Don't forget to do the proper math.

https://www.hotrod.com/articles/cali...-speedometers/

https://www.tciauto.com/speedometer-gear-calculator
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Old May 6th, 2019, 07:46 AM
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thanks, any suggestions on the easiest way to determine what rear end ration I have? I doubt it's original.
Steve
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Old May 6th, 2019, 10:44 AM
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Put a mark on the tire, a mark on the driveshaft and see how many rotations of the shaft per one turn of the tire.
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Old May 6th, 2019, 12:13 PM
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thanks, that sounds easy enough. Before i dive into a black hole researching possible ratios on the internet, what are the most common ones?
Steve
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Old May 6th, 2019, 12:15 PM
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And how do I take the number of driveshaft rotations and calculate the ratio?
Steve
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Old May 6th, 2019, 12:21 PM
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Originally Posted by frankr442 View Post
Put a mark on the tire, a mark on the driveshaft and see how many rotations of the shaft per one turn of the tire.
Too much error that way. 10 turns of the tire is better to determine accurate driveshaft revolutions. Too easy to mistake 3.08, 3.23, 3.42 doing only one revolution. but with 10, it's easy. Sounds like the rear is closer to a 3.91 or 4.33 though. Standard rear end on that car should be either 2.56 or 2.78
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Old May 16th, 2019, 06:21 PM
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Originally Posted by don71 View Post
Yes Steve you sure can. You'll likely be changing both the drive gear and driven gear, depending on which housing you currently have. If you're lucky, you'll only be swapping the driven gear but you have pretty big spread of inaccuracy. Don't forget to do the proper math.

https://www.hotrod.com/articles/cali...-speedometers/

https://www.tciauto.com/speedometer-gear-calculator
You can avoid a drive gear swap by using an external reducer. When I changed from a 2.78 to a 3.90 ratio, I used an external reducer in combination with a different driven gear in my Jetaway I was running at the time. Since then, I swapped the Jetaway for a 200-4R and no longer needed the external reducer and with the correct driven gear my speedo tracks actual (via GPS) exactly, other than a 4 MPH offset that stays constant regardless of speed. The only way to fix that offset is to remove the needle on the speedo and move it by 4 MPH (unless there is a trim adjustment on the back of the speedo for this).
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