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Missing Speedometer Gears
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Tobias View Drop Down
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    Posted: Aug/20/2018 at 11:42am
Hi everyone!

I'm trying to find out which speedometer gears I have to use for my 1979 Pacer (258 cid, BW SR4 4-speed manual transmission, 2.53 rear axle). The previous owner had to repair the transmission, but lost both speedometer gears, i.e. drive gear AND driven gear. Angry
The 1979-83 Factory Parts Catalog lists three drive gears (8/9/13 tooth) and three driven gears (20/21/27 tooth) for this engine/transmission combo. I just don't know which set of gears would be the right one.

It seems the 1970 TSM had a speedometer gear chart in the Electrical Section, but I couldn't find a similar chart in my 1978 TSM:
http://theamcforum.com/forum/forum_posts.asp?TID=76128&PID=687753&title=borg-warner-m12-automatic-speedometer-drive-gear#687753

I also tried some of the online speedometer gear calculators, but the only combination that comes close to an accurate reading would be 13-tooth drive gear and 27-tooth driven gear, but as the calculated number of driven teeth would be ~26 the speedometer would still show ~4 % too SLOW if I'm correct. Seems strange to me for a factory supplied solution!
Is that really the best solution AMC came up with? Or did I overlook something?

Tobias


Edited by Tobias - Aug/20/2018 at 11:46am
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-4% is well within the transportation industry standard (not regulation) of - 10%to +2% of the day. Tire wear and inflation pressures will throw off a speedometer 4% or more.
Even today the most stringent standard is Europe with nothing 110% higher at any speed.
I believe GPS's and radar detectors are skewing the public perception of what is build-able accuracy with so many factors, wear, pressure inside and outside the tire, temperature... sure there is more.
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Tobias View Drop Down
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I was just wondering why you would want to already start with 4 % deviation. As you mentioned tire wear and inflation sure add to it and the indicated speed might be even further off.
I don't know about the rest of Europe, but in Germany speedometers aren't allowed to show less than the actual speed.
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Back in the 50's and 60's when car magazines did comprehensive test some usually had a speedometer error test.
Most erred,  and when they did, it was always registering higher than what it was actually going.
Some were as high as 10%, a few even higher.

I checked about a dozen different test, not one registered a lower speed than it's actual.
66 American SW, 66 American 2dr, 82 J10, 70 Hornet, Pound, Va.
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You cannot have a speedometer that reads actual speed unless it has a reference other then the tire rotation.
Again, tire temperature cold start to on the road for an hour, altitude, air temperature, tire wear, the list goes on...
Even GPS and radar have tolerances!
If anyone states their manufacturing a speedometer that reads correctly 100% all of the time I would challenge that! 
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Points: 7544 		Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Aug/20/2018 at 9:45pm
if you have gears that will work, even with an error, i'd just install them, get the car on the road, calculate the error, and adjust the cable end, driven gear accordingly. it might just be easier! there's so much trivia to sort out to get a car back on the road!
1960 Rambler Super two-door wagon, OHV auto
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http://www.ramblerLore.com
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Thing is, I don't have the gears yet. And it seems they aren't very easy to come by (especially being in Europe). So as a starting point I thought it would be nice to know what to even look for. Wink
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Any chance this place in Berlin, Germany may be able to help with getting parts ?
The Speedometer gears for that BW Trans may be the same as gears used in some other transmission.


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The 70 manual shows a 2.37 ratio, that uses a 9 drive, 17 driven for most tire sizes.

It also shows a 2.73 ratio, that uses a 8 drive, 18 driven for all tire sizes.

Your 2.53 gears are roughly in between the two ratios. So either of the two above should get you sort of close.

For your situation, I would personally go with a 8 drive, 17 driven. Try it & see how far off it is. Calculate the percentage of error, then change the tooth count by the same percentage. More teeth will slow it down, less will speed it up on the driven. That will get you as close as possible.

I suspect that transmission uses Ford style gears. Here is a set of driven gears, gives you options to play with: https://www.ebay.com/itm/Ford-SPEEDOMETER-GEARS-5pc-SET-Kit-GENUINE-NOS-Fairlane-MUSTANG-Transmission/142551616578?hash=item2130bd0c42:g:WSUAAOSwcQlZt2fD
I have personally used those gears in my AMC.


As others have said, you are never going to get the speedo to be 100% accurate. The best you can hope for is fiddling around with it to get it as close as possible at the speed you cruise at most often. That may make it off at other speeds.

I drive a 2013 F250 that is 10% or more off (it reads fast) above 40 MPH. That's with all the latest & greatest technology.
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Originally posted by PHAT69AMX PHAT69AMX wrote:

Any chance this place in Berlin, Germany may be able to help with getting parts ?
The Speedometer gears for that BW Trans may be the same as gears used in some other transmission.

http://automatikgetriebe.com/index. - http://automatikgetriebe.com/index.html
Thanks for the link! I did not know that place and will contact them to see if they can help.

Originally posted by scott scott wrote:

The 70 manual shows a 2.37 ratio, that uses a 9 drive, 17 driven for most tire sizes.

It also shows a 2.73 ratio, that uses a 8 drive, 18 driven for all tire sizes.

Your 2.53 gears are roughly in between the two ratios. So either of the two above should get you sort of close.
That's just the info I was looking for! Based on my calculations I came up with an 8-tooth drive gear and a 16-tooth driven gear for a near perfect fit. I was just wondering why AMC didn't offer that combination and thought I might have missed something.

Originally posted by scott scott wrote:

I suspect that transmission uses Ford style gears. Here is a set of driven gears, gives you options to play with: […]
That's right. For all I know the BW SR4 transmission was also used in Gen 3 Ford Mustangs and
Ford Light Trucks of that era. I think it even has a Ford part number cast into the housing.
I found this Ford Light Trucks speedometer gear catalog that lists Ford part numbers for the gears they used on their transmissions (including a "4/M/T" that should be the SR4):
http://www.icsarchive.org/icsarchive-org/private/Ford%201980-1989%20Truck%20Master%20Parts%20Text/21%201980%20fotpt-speedometer.pdf - http://www.icsarchive.org/icsarchive-org/private/Ford%201980-1989%20Truck%20Master%20Parts%20Text/21%201980%20fotpt-speedometer.pdf

Lo and behold – they also list a 16-tooth driven gear! So I'll try that combo and see how far off I am.

For documentation purposes here's a list of the Ford part numbers that I took from the catalog. (Just take it with a grain of salt because I could not verify yet that those are indeed correct for the SR4 transmission!)
Speedometer Drive Gear
Ford (1.375" ID, Ball Retainer or Spring Clip, Right Hand Thread)
(Green-6 Tooth)        D0TZ-17285-A
(Blue-7 Tooth)        E0TZ-17285-B
(White-7 Tooth)        E3TZ-17285-C
(Yellow-7 Tooth)        E7TZ-17285-B
(Brown-8 Tooth)        C8OZ-17285-B
Speedometer Driven Gear
(16-tooth)    C0DZ-17271-A
(17-tooth)    C3DZ-17271-C
(18-tooth)    C0DD-17271-B
(19-tooth)    C0DD-17271-C
(20-tooth)    C1DD-17271-A
(21-tooth)    C4OZ-17271-A
The gear set on eBay you found looks very similar at least, but the part numbers don't match. And as far as I know the SR4 gears cannot be used in the automatic transmissions that are listed in the description. So that might not work.
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