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

 

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

 

thanks, any suggestions on the easiest way to determine what rear end ration I have? I doubt it's original.
Steve

 

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.

 

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

 

And how do I take the number of driveshaft rotations and calculate the ratio?
Steve

 

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

 

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).