speedometer problem - rear end gear ratio?

Old March 28th, 2019, 01:21 PM
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speedometer problem - rear end gear ratio?

I know possibly I'm in the wrong category but. I purchased a 69 Cutlass a few months back that has had some mods done to it. At low speed the speedo is accurate but the faster I go the more it is out. At 60 mph it reads 75-80. The tire are slightly smaller than original but I doubt that is the issue. A prior owner pulled the 350 out and put a 455 in so I wonder if he swapped rear end gears. At 60 mph it sounds like I'm at a high rpm. It shifts through all 3 speeds. I recently purchased a tach to confirm the rpm but at the moment it is in a body shop being stripped. The paint was lifting. It must have an easy 6 coats of paint on it. We even found a layer that had flames painted on it. I believe there is a # on the axle shaft but if he changed the gears that wouldn't matter but, even with a gear change would the speedo be that far off?

thanks Steve
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Old March 28th, 2019, 05:41 PM
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Originally Posted by Phoenix8990 View Post
At low speed the speedo is accurate but the faster I go the more it is out.
yep, that's how it works. The speedo will be off by a percentage, so at 0 MPH it will read correctly, but the faster you go the more it will be off.

Originally Posted by Phoenix8990 View Post
At 60 mph it reads 75-80.
You can use that to calculate the percentage error, then use that to determine the actual speed.


Originally Posted by Phoenix8990 View Post
even with a gear change would the speedo be that far off?
Yes. I swapped the original 2.56 rear for a 3.55 rear and the speedo read around 80 when the car was doing 60.
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Old March 29th, 2019, 07:15 AM
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tire size

Tire size does affect the speedo also. Smaller tire will give steeper ratio increasing the speedo MPH spinning the cable faster and more diameter tire will give less ratio or slow down the speedo
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Old March 29th, 2019, 07:21 AM
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Here's a good calculator to use to find out how much the different size tires effect your speedo.

https://tiresize.com/comparison/
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