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[79dodge4ever]

the 66 dart has had a 4:11 posi rear end in it since me and my father started working on it and the orginal drive gear in the rear of the transmisson for the speedometer is making the speedometer read too low, says 60 miles an hour when you might be doing 40-45 how do you determine what one to use to get the speedometer to read right?

[Dippy.org]

[M_Body_Coupe]

Here are a couple of handy links, you need to know the rear end ratio (which you do) and the size of the tire (how tall is it), the chart then gives you the tooth count of the speedometer gear to buy.

http://www.yearone.com/yodnn/tech/TechFAQ/TransmissionFAQ/MoparSpeedoGearChart/tabid/415/Default.aspx

http://home.earthlink.net/~mopared/speedo.htm

http://www.tciauto.com/Products/TechInfo/speedo_gears.asp

[DobaMark]

You can also drive a measured distance and calculate the percentage your speedmoter is off and then buy a gear with that percent more or less teeth.

[dodge97318j]

the speedo gears ive seen have the gear ratio they work with scribed on them. might be color coded too, but i am not sure about that.

[79dodge4ever]

thank you vey much M_Body_Coupe that anwsered my questions, just wont be able to change the drive gear for a little whileit in a very small building till april. I have asked that question to many people even my collage instuctor and he went blank! he didnt have a clue!

[M_Body_Coupe]

You are in luck given the age of the car...the '66 will use the old style speedo gear, which is basically a steel shank with plastic/nylon gear mounted on it...these can be found fairly easy, what is pretty darn near impossible to find is the newer style (which our M-bodies use) plastic snap-over speedo gear...it took me, and I kid you not, about 2 yrs to source one with 37 teeth...eventually got 2 of them, from an old Mopar dealership out in the middle of nowhere...cost: $5 to my door, best deal ever! 

[DobaMark]

I still think this is your best method.  It calculates the error without guessing on tire sizes, because tire height vs. rolliing diameter can vary by tire design (radial or bias for example).

Find a measured known distance or use a GPS and drive 10 or more miles (more is better).  Record how many miles your odmoter registers over that distance.  Count your old speedometer teeth.  Then use this formula:
New Gear teeth = Odometer miles x Old gear / Known Miles

When I had my 67 Valiant, I used the charts and it still wan't that accurate with the new gear.  Then I used the calculation method and ordered a different gear and it was accurate.

So, if you have a 30 tooth gear and you speedometer reads 12 miles over a 10 actual mile course:
New gear = 12 x 30 / 10
So in this example, you'd need a new 36 tooth gear.

[Erics5th]

A tip for any M body owners with cruise control. A speedometer cable with the old style adapter for the pinion gear can be used from a pre '80 Cordoba/Charger/Magnum. Fits perfectly..I had the same problem when the previous owner installed a 2.94 to 1 8 3/4 rear into my 5th. An R body speedo cable/pinion adapter may work also.

[Ark]

http://www.mymopar.com/speedometer.htm

Knew I had that stashed in my bookmarks for a reason.