Identifying 66 Charger rear end gears?

[Terry Kitzman]

The 66 Charger I bought has a very inaccurate speedometer, It reads about 10 MPH slow.
Is there a tag or number on the third member to specify ratio?

 

[brad 66]

there is normally a tag on differental bolts

 

[Don Frelier]

If the gears were changed it likely has no tag, or the tag is incorrect.
You can get a rough number by doing this:
Jack up one rear wheel, car in neutral, put a chalk mark or painters tape on the driveshaft and on the tire.
Rotate the tire 10 full rotations = T and count the driveshaft revolutions = DS.
DS X 2 / T = rough ratio.
So if DS = 16 then you probably have a 3.23.

The more tire revolutions the more accurate your number will become.
If you have not changed the tire size greatly, then my guess would be someone changed from a 3.55 to a 3.23.
Since 3.55/3.23= 1.099.

 

[Charger Dana]

Or pull the pumpkin and check the ring gear. Or pretend you didn’t know...

In Canada we’re in kilometres and I have had 4 different gear sets in, so I use a free app, Speedbox, to figure out rpm/gear/speed and then basically drive by tach.

There are lots of apps, but if you’re in a hurry download this. Put it on, sit it on your console, works fine. You can rough calibrate to your speedometer.
https://itunes.apple.com/us/app/speedometer-speed-box-app/id307529605?mt=8

Once you figure out the gears, you can install the correct pinion:
http://www.brewersperformance.com/products.php?cat=277

Also check tire size- I have used 25.5-28”, all kinds of variation affects speedometer speed.

 

[R/T Boy 67]

If sure grip, jack up both wheels, turn wheels one turn and count driveshaft revolutions. If open diff. Leave car on ground and push car one wheel revolution and count driveshaft revolutions. Two people are handy for this. 3 1/4 turns 3.23. 3 1/2 turns 3.55 etc.

 

[R/T Boy 67]

If you know what percentage your speedo is off by, then gear ratio, tire size etc. is irrelevent. Just change the speedo gears teeth number by the same percentage, either up or down, depending which way the speedo reads. Technically you
should set the gears so the odometer is right, then have the speedo recalibrated if it is off. How you do it is of course up to you.