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Unread 08-12-2013, 04:46 PM   #1
natasharomanoff
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1980 CJ7 
 
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Speedometer Gear

My speedometer is off by more than 10mph slow. I have 3.54 gears in my "N" coded AMC 20. The 265-75-R16 Procomps measure out at 32" so I go to the speedometer chart on the Quadratec website and they don't even list a 3.54 ratio as an option, so I look up for 3.55 close enough, then slide over to the tire diameter of 32" only to discover nothing there. So now what? I need to get the speed right but it seems no one makes a gear to correct the speedometer. Is there another or better source I haven't discovered yet? I'd change to a 26 or 28 gear to close the gap but I don't know how much too fast the gauge would read if I go that route. Rational suggest I need a 31 tooth gear since there's a gap in the chart between 26 and 32 tooth gears.
Suggestions?

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Unread 08-12-2013, 06:05 PM   #2
adman02
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The speedo connects to the transfer case. And, for some reason that I'm sure someone else will explain, changing the speedo gear in a dana 20 is difficult to impossible. If you had a Dana 300, then you'd be fine.

I looked into this a while ago and I think you can use a ratio adapter to correct things, but I remember it being kind of expensive.
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Unread 08-12-2013, 06:29 PM   #3
chatham
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years ago speedometer shops could provide a adapter that connect to the transfer case and then to the speedometer cable. this adapter would adjust the gear ratio and get you the correct speed. so you might look into this.
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Unread 08-12-2013, 06:34 PM   #4
LeePrather
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I believe you need a ratio adapter. It screws into the Dana 20 and the speedometer cable screws into the adapter. There are places that you can call and give that info to and they will determine the gearing of the adapter...

Search "ratio adapter"

http://www.gaugeguys.com/speedcorrect.htm
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Unread 08-12-2013, 06:45 PM   #5
Spieg8
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Quote:
Originally Posted by adman02 View Post
The speedo connects to the transfer case. And, for some reason that I'm sure someone else will explain, changing the speedo gear in a dana 20 is difficult to impossible. If you had a Dana 300, then you'd be fine.
I think the OP was talking about the gear ratio in a AMC20 rear differential, not a Dana20 transfer case. I'm assuming that on a 1980 CJ we are talking about a Dana300 transfer case?

The charts for speedo gears are mostly wrong from what I've seen. Also I would not assume what gears you have just from a stamp on the case... a previous owner could have changed gears.

Here's the method I use to figure out proper speedo gear;

If you have a GPS it’s pretty easy to calculate... You only need three things: Tooth count of speedo gear that is currently installed, Speedometer reading, and GPS reading. Run it up to 55 or so and record speedometer reading and GPS reading.

speedometer reading / GPS reading = X
current tooth count * X = accurate tooth count
-Or-
speedometer / GPS * current tooth count = accurate tooth count

______________________________________

Example 1 –
60 mph on speedometer and 65 mph actual on GPS. You need to speed the speedometer up so you need a gear with fewer teeth.

60/65 = .923
Multiply this by your current tooth count (34 in this example).
34 x .923 = 31.38

A 31 tooth gear would be the closest. Speedometer would read just slightly faster than actual speed.
______________________________________

Example 2 –
65 mph on speedometer and 60 mph actual on GPS. You need to slow the speedometer down by installing a gear with more teeth.

65/60 = 1.083
Multiply this by your current tooth count (34 in this example).
34 x 1.083 = 36.83

A 37 tooth gear would be the closest. Speedometer would read just slightly slower than actual speed.
______________________________________

For my Jeep it worked out as;
56/50 x 28 = 31.36 (went with a 32 count gear… rounded up because tables suggested either 33 or 34 tooth gears)
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Unread 08-12-2013, 07:46 PM   #6
adman02
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I was responding to this...

Quote:
Originally Posted by natasharomanoff View Post
My speedometer is off by more than 10mph slow...So now what? I need to get the speed right but it seems no one makes a gear to correct the speedometer. Is there another or better source I haven't discovered yet?
Changing R&P just to get the speedometer to be more accurate seems like a lot of unnecessary work and expense.

Maybe I'm not understanding the OP's question.
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Unread 08-12-2013, 08:10 PM   #7
LeePrather
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Quote:
Originally Posted by adman02
I was responding to this...

Changing R&P just to get the speedometer to be more accurate seems like a lot of unnecessary work and expense.

Maybe I'm not understanding the OP's question.
It's a t150 with a Dana 20, so what you and I advised is correct...ratio adapter on transfer case
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Unread 08-12-2013, 08:15 PM   #8
Spieg8
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Quote:
Originally Posted by adman02 View Post
I was responding to this...



Changing R&P just to get the speedometer to be more accurate seems like a lot of unnecessary work and expense.

Maybe I'm not understanding the OP's question.
Don't think anyone is suggesting a change of Ring & Pinion.
The OP was just trying to use the R&P ratio (AMC20) and tire size (32") to estimate the proper speedo gear per the chart at Quadratec.

I did dig up a more complete listing of speedo gears that may be of some use. If Quadratec doesn't have what you need, there may be other sources.

Teeth Color Part Number
24 WHITE 3410024
25 BLUE 3410025
26 RED 2538926
27 WHITE 52068149
28 BLUE 52068150
29 BLACK 52068151
30 YELLOW 52068152
31 GREEN 52068153
32 BLACK 52068154
33 YELLOW 52068155
34 GREEN 52068156
35 ORANGE 52068157
36 RED 52068158
37 WHITE 52068159
38 BLUE 52068060
39 GREEN 52068061
40 ORANGE 52068062
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Unread 08-12-2013, 08:17 PM   #9
lucdog
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..

. http://www.transmissioncenter.net/sp...n_______va.htm

Scroll down 1/2 way on the page.
This is what your looking for.


image-3383268297.jpg


B
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Unread 08-12-2013, 08:25 PM   #10
Spieg8
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Quote:
Originally Posted by LeePrather View Post
It's a t150 with a Dana 20, so what you and I advised is correct...ratio adapter on transfer case
Sorry for the confusion, I didn't read the profile. You are correct then... Dana20 speedo gear is not interchangeable. Given that the OP was searching the speedo gear tables, I assumed that he was just having trouble finding a vendor with a 31 tooth speedo gear for a Dana300. Although, this is probably the first time I've heard of an '80 CJ with a Dana20 as Dana300 was standard equipment 80-86.
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Unread 08-12-2013, 08:46 PM   #11
LeePrather
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Yeah I was kinda surprised too and had to look to confirm
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Unread 08-12-2013, 09:11 PM   #12
natasharomanoff
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Yes, I was looking for a new speedometer gear to correct the reading. It's a T150 trans and Dana 20 Transfer. I think that's the only transfer which will work with that transmission from what I've read. The engine code says the 258 is a 1976 model so I bet the drivetrain and possibly frame are from that era. the VIN number says it's a 1972 CJ5 but the only thing CJ5 about this Jeep seems to be the VIN. My bad for not researching things deeper before I bought the thing.
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Unread 08-12-2013, 09:14 PM   #13
natasharomanoff
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Yeah I'm not replacing the R&P until I build the whole thing properly. I was using the tire diameter and "original" R&P numbers trying to find the right speedo gear.
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