70 Challenger speed o reading high

[nctoymaker]

Another of the many issues needing resolution, my speedometer is reading a speed that is much high than the travel speed.

Just had the rear rebuilt and since the speedo was registering a higher speed the shop suggested a lower gear ratio.  Originally the rear had a 3.23:1, the shop put in 2.76 gears.  Didn't help, speed o is still showing to high a speed.

Car is 383, 727 automatic.  Tires are P245/60 14" that gives me 25.6" diameter.  Having a 2.76 rear I found I should have a 29 or 30 tooth speed o gear in the tranny.  I put the existing gear (see photos) and is a 25 tooth?

Can anyone help me as to which gear would work best (29 or 30 tooth) for the combination tire/rear gear ratio I'm running? And, Can anyone share where to purchase at a fair price?

Also looking at the photos I note the design of the existing gear is different from ones I've seen on line.  Note the bell/orange cone design on the plastic end.  The ones I've seen on line do not show such a large bell shape.

Any help ordered would be greatly appreciated

Bob (nctoymaker)

[Cuda Cody]

@nctoymaker  this thread has a lot of good info and a chart to help you pick a good speedo gear.  @MoparDave  should be able to get you any of them and let him know you're with the E-Bodies group so you get the discount. 

[Shane Kelley]

29 should get you real close.

[Shane Kelley]

Chart from Roseville Moparts site.

[nctoymaker]

Thanks guys, I'll try and make contact with Mopar Dave.

[Burdar]

New speedo gears are extremely expensive.  Check Ebay.  There are people selling nice used gears.  Check to see if they have the gear you need.

[Burdar]

Instead of just going by the manufactures tire specs, try this...

Calculate the exact "loaded" tire diameter by measuring the "rollout".  You can do this in your driveway if it's flat.  Start by having your valve stem in the 6:00 position.  Put a mark on the ground where the valve stem is.  Then roll the car forward in a straight line until the tire has made one complete revolution.(valve stem is at 6:00 again)  Put a mark on the ground and then measure that distance.  What ever your measurement, divide that by π.(3.141593)  That will be your exact tire diameter.  The actual diameter and the manufactures specs can be way off.

[Katfish]

[Burdar]

My original 28 tooth gear is light blue...not white like the first chart indicates.

[6bblgt]

GG also lists 28 tooth: 2538928 as "blue" (38 tooth is also "blue" 2538938)

& 24, 27, 37, 42 as "white"

[RUNCHARGER]

If the chart works out to a 28 your speedo should currently be reading 10% high, is that what you're seeing?