Speedometer Drive Gear

[John Cason Fudala]

When driving my 63 the speedometer reading is about 5 to 7 faster than what I am actually traveling. I have not removed the gear to count the number of teeth it has. The rear axle ratio is 3.21. Does anyone know how many teeth the gear should have at that ratio? And if is is incorrect, where can I source a new gear? Also, does anyone know of a chart that shows the number of teeth required for a given ratio? I am running Hankook Optimo H724 235-75R-15 tires. Thanks

[Jason Edge]

If your 63 Cadillac has the 3.21 differential (stamped 1) you need the 20 tooth gear.
If you had the 2.94 differential (stamped 2) you would need the 18 tooth gear.
This can all be found in our Rear End Differentials: Gear Ratios, Controlled Differentials and S...  Help Page.

I should have the 20 tooth gear. Email me at [e-mail]
Jason. 

[Russ Austin]

Your car came with bias ply tires, and now you rrun radials that are not the exact size.  I think the tire size is causing the speedometer to be off. 

[Jason Edge]

As stated in our  speedometer effect section of our Tire Help Page, using the one rotation circumference of the tires, the affect of running the most common replacement radial 2.35/75R15 vis the standard 8.2-15 bias ply is about 2.4%. 

Simply multiply your shown speed by 0.976 (1 - 024) to get the actual mph. So 50 mph your speedometer would only be 1.2 mph off; at 75 mph 1.8 mph off, and at 100 mph 2.4 miles per hour off.

So even giving some minor variance, it points to the wrong speedo gear.   Of course if you are now running a very small tire (like  some 205's I saw on one parts car) you can begin to approach that 9% mark. I pulled on a parts car with 195's and cut springs one time and it was sitting almost flat on the ground!  That speedometer was probalby way off.