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Thread: 74 Speedo Gear
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Crit
Revvin Up
Finally REPUing
Posts: 90
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posted December 18, 2009 11:30 PM |
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74 Speedo Gear
I just went from a 3.30 rear to the 4.63 gearset in a 74 truck, and I'm using an SA RX-7 gearbox. I've asked around and found a guy with a few REPU speedo gears, but I don't know how to tell a 74 speedo gear for a 4.63 rear from the rest.
Are there any identifying marks, or does anyone know what the tooth count should be?
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77 REPU
84 GSL-SE
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Klaus45
Redlining
On two wheels
Posts: 218
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posted December 19, 2009 11:32 AM |
 Edited By: Klaus45 on 19 Dec 2009 11:51 |
Might be time to talk to Mazda...
If you can locate a knowledgeable, helpful old-school parts man(ager?)... and cross-reference part #'s for various speedo gears... (?)
From the '74 REPU Parts Manual:
Part # 0862 17 341 Gear, Drive - Speedometer
From the '74 REPU Shop manual:
Manual Trans. (4-speed) Gear Ratios
First 3.683
Second 2.263
Third 1.397
Fourth 1.000
Reverse 3.692
From a listing of US spec Mazda Driveline Ratios:
'74 REPU Rear Axle: 4.625 Net Final: 4.625
'82-'84 B2000 : 3.308 Net Final: 2.894
'79-'83 RX-7 : 3.909 Net Final: 3.227
Hope that helps!
(Perhaps contacting Racing Beat, Mazdatrix, Atkins, Pineapple, etc. would prove helpful?)
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Brad
Rotorhead
Posts: 1672
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posted December 21, 2009 09:44 AM |
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The SA tranny uses a plastic speedo gear that is coarse. I imagine the gear on the output shaft it meshes with is also coarse. It sounds like it's not interchangable with the one you need with a 4.62 rear gear which is a finer thread.
http://solomiata.com/Drivetrain.html
The more teeth on the plastic speedo gear, the more it'll slow down the speedo to compensate for shorter rear gear and shorter rear tires. I had a SA tranny, 17 tooth and 4.62 and my speedo read much, much faster than reality.
I have a 21 tooth white plastic speedo gear that came from a 4.62 REPU. Haven't gotten it to work because the shop that rebuilt my tranny didn't put the metal speedo gear on the output shaft! That's my next project.
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Crit
Revvin Up
Finally REPUing
Posts: 90
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posted December 26, 2009 09:50 AM |
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From everything I've read, the SA uses a coarse drive gear on the output shaft. I'm not sure what the ribcases used, but I was hoping it was a coarse drive gear which meant I could move a 74 speedo gear housing from an old ribcase and pop it into the SA tailshaft. Brad, are you saying that the ribcase drive gear is metal? Any idea if it's coarse or fine?
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77 REPU
84 GSL-SE
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kansei
Redlining
Wankelized
Posts: 423
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posted July 23, 2010 12:51 PM |
Edited By: kansei on 23 Jul 2010 12:52
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Sorry to jump in here...and late, but...
...but I'm just curious where the 3.30 ring and pinion came from? And I'm assuming you swapped in the 4.62, and just rounded to 4.63, yes?
I'm trying to dig up the info to back it up, but IIRC, an SA plastic speedo gear (not the one on the output shaft of course) essentially interchanges with the one from a '74/5 4 speed REPU trans. They have a different number of teeth, but will mesh with the gears on the output shafts of the two transmissions. Again, IIRC. When I was doing research on this topic for my green '74 a few years back, I seem to recall coming across some info about the internal gearing in the speedometers themselves being different between the years, and even different between auto and manual trans equipped trucks, but I can't find that right off at the moment. It sticks out in my memory because my truck was a '74 chassis, but it had a '75 or '76 cluster (from an auto) and SA trans in it when I got it. I had to source a true '74 manual trans cluster, and Mike was kind enough to help me out with a '74 4 speed trans to help with the speedo gear and any other bits I needed to get the speedometer to read right.
Neal.
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Crit
Revvin Up
Finally REPUing
Posts: 90
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posted July 23, 2010 03:15 PM |
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The 3.30 came from a diesel B-series and commonly swapped into V8 swaps when keeping a Mazda axle. Since posting this, I've finally found a very late SA tailshaft. It's got the SA shift tower location but the rods and forks on the gearcase are FB-style with the FB selector axis. I've now got just this one tailshaft that will let me cleanly run any FB gearbox in the truck and even keep my old shifter. This opens up all the 626 / Miata / MPV / RX-7 speedo gears. I'll just count my gear when it all goes together, pace out the speedo, and go hot the junkyard for whichever gear will work best.
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