speedometer??

[Powerhouse]

I put a 39 Chrysler speedo in my 39 plymouth with a 36 chrysler transmission and the speed registers at about half of what it is. Any reason why? I completely rebuilt the speedo so it's definately working properly.

[55 Fargo]

I put a 39 Chrysler speedo in my 39 plymouth with a 36 chrysler transmission and the speed registers at about half of what it is. Any reason why? I completely rebuilt the speedo so it's definately working properly.

Okay, so you have used the 39 Chrashler speedo, and 36 Chrashler Tranny.

What is the gear ratio of the diff, what size of tires are you using?

The trans also has a drive gear ie, 17 t or 19 t, you need to know what you have in there.

Usually the important factor in speedo accuracy is this, the correct speed gear as to the ratio of the diff.

Tires will play a role in speedo accuracy too, if the the tire is tall, it can throw things out, as well if they are short.

How about the odometer, can you go for a 10 mile ride, and see what the odometer goes up to, should be close to 10 miles, if all is well, there can be a slight margin of error.

Let us know your current diff ratio, tire size, etc, see if we can help................Fred

[Powerhouse]

oh boy...

i guess i'lll try that tommorow...take out the tranny speedo gear...should it say something?

[Jim Yergin]

The 1936-1948 Plymouth Parts book that Chet has posted on his web site (http://chetscoins.com/1948p15/DocumentWeb/) has a chart on page 314 that shows what speedometer drive gear (16, 17, 18 or 19 teeth)to use depending on tire size and rear end ratio. I assume the same info would apply to Chryslers. It also has part numbers. Even if your drive gear does not have a part number on it you could just count the teeth and determine if it is the correct one. You might want to check it out. The password to gain entry to the document is "password"

Jim Yergin

[Powerhouse]

WOW THANKS...that's really neat.

I have 3.9 rear end soooo...

3.9 rear / 16x650 tire / #652846 pinion(what's that?) / 16 teeth

I was running with a 19 teeth and now have a 18 teeth from my other tranny in there.

ANYONE have a 16 teeth gear????

[Powerhouse]

SWELL...thanks!

[Jim Yergin]

According to Partsvoice, Collectors Auto Supply (http://www.collectorsautosupply.com/) has the 16 teeth gear (part #652846).

Jim Yergin

[Powerhouse]

nea thanks...

does anyone know if I need the gear with 16 teeth that comes out the side AND the internal gear that goes on the shaft?

[Jim Yergin]

You only need to change the gear that the speedometer cable goes into. The internal drive gear is on the output shaft of the transmission and is the same regardless of axle ratio or tire size.

Jim Yergin