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904 Speedo Gear Replacement?
https://slantsix.org/forum/viewtopic.php?t=25189		 Page 2 of 2
Author:  SlantSixDan [ Sat Oct 06, 2007 7:52 am ]
Post subject:  Re: gear size
VDart, there appears to be a misprint in your '64 FSM or a transcription error.

'64 FPC (A904 transmissions/7-tooth output):
6.50-13 2.93 17 orange
3.23 19 dk blue
3.55 20 green
3.91 21 grey
7.00-13 2.93 17 orange
3.23 18 dk purple
3.55 20 green
3.91 21 grey
7.00-14 2.76 16 brown
2.93 16 brown
3.23 18 dk purple
3.31 18 dk purple
3.55 19 dk blue
3.91 21 grey

Boldface line above is closest match for the OP's tire size and gear ratio. The 1965 FPC does not list 7.00-14 as an available tire size, because tire sizes changed in '65. The old 7.00-14's new equivalent was 7.35x14, and the old 6.50-14's new equivalent was 6.95x14. The '65 FPC chart calls for a 20-tooth green pinion for both 6.95x14 and 7.35x14. Checking this chart, we see that a 7.00-14 is most closely equivalent to P205/70R14, which is the size the OP is running. We also see that the difference between 6.95x14 and 7.35x14 is the difference between a P185/75R14 and a P195/75R14 or between a P195/70R14 and a P205/70R14, a difference not large enough to call for a different speedo pinion.

So, either a 19- or a 20-tooth pinion will get the OP's speedometer into the ballpark of accuracy. A 19-tooth will drive the speedo a little faster, a 20-tooth a little slower. The tire size equivalences in charts like the one above are approximations, but given that the P205/70R14 is on the larger-diameter side of the 6.95/7.35 split, a 19-tooth pinion is probably the best pick. Selectivity with the '60-'65 pinions isn't anywhere near as fine as with the larger '66-up pinions.

But an 18-tooth pinion is definitely not the right one for this application.
Author:  VDART [ Sat Oct 06, 2007 9:32 am ]
Post subject:  speedo stuff
Thanks Dan-- I changed the ratio-- it was 3.31 not 3.55

This all came about -- I have a spare blue gear-- I counted the teeth and came up with 21-- it does not match any of the listings--- I'm going to count my natural one & see if matches the 17 listed--
I did not go through the manual 3 & 4 speed gears-- my list was for the auto gearing from that tsm. the 62 manual lists t-flite 6 seperate from the other auto-- whereas the 64 combines them.

I have also seen height differences in tire manufacturers -- same rim application

Lawrence
Author:  SlantSixDan [ Sat Oct 06, 2007 10:44 am ]
Post subject:  Re: speedo stuff
Quote:
I have a spare blue gear-- I counted the teeth and came up with 21-- it does not match any of the listings
Perhaps the dye in the pinion has reacted over the years with the transmission fluid.
Quote:
The 62 manual lists t-flite 6 seperate from the other auto-- whereas the 64 combines them.
The '64 manual separates by A904 vs. A727. That's because starting in '64, there were V8/904 applications. From '60-'63, the A904 was only used with 6-cylinder engines, and the A727 only with V8s.
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