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Old 12-17-2014, 02:45 PM   #1
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Default A different Speedometer gear question?

Iím fiddling with rear end ratios and it would be nice to keep a speedometer that would be reasonably accurate if possible.

Using a Quaife wide ration transmission and will be swapping between 4.778 and 3.909 rear end ratios for different disciplines of competition.

Typical speedo gears I know can be found with tooth counts between 17 and 23 teeth. A 21 tooth works ok with a 3.909 but I think it would take a 25 tooth that doesnít exist to work with the 4.778.

Wondering if there is different drive gears inside the transmission that would fit to make it work. Stock Miata drive gear inside the transmission is M503-17-341A, it has 6 teeth. I have a 1984 B2000 transmission apart and the drive gear in it fits the Miata and it has 7 teeth and basically the same dimensions but the helix angle on the teeth goes the opposite direction.

By my calculations if I could find a part number for a 7 tooth drive gear with the correct helix direction I could get reasonable speed readout with either a 4.778 or a 3.909 rear end on the stock speedometer by swapping between an 18tooth speedo gear for the 3.909 and a 22 tooth with the 4.778.

It may be a stretch but does anybody know a part number for a 7 tooth speedo drive gear that would fit in a Miata transmission and have the helix direction correct? Late 80ís RX7ís use some different part numbers even have a different one between turbo and non-turbo for that part while but the parts books donít list tooth count or helix direction so I have no Idea if they would work without buying a bunch of different ones.
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Old 12-17-2014, 03:44 PM   #2
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Use a gear for 3.9 and slow it down electronically when using the 4.77, bluntly assuming NB stuff
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Old 12-17-2014, 04:02 PM   #3
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Use a gear for 3.9 and slow it down electronically when using the 4.77, bluntly assuming NB stuff
NA mechanical speedo.
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Old 12-17-2014, 04:36 PM   #4
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Use math or GPS to figure out your actual speeds.

Email RevLimiter

Get a custom gauge, instead of MPH/kph, get one that's for 3.909/4.778
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Old 12-17-2014, 04:45 PM   #5
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Originally Posted by curly View Post
Use math or GPS to figure out your actual speeds.

Email RevLimiter

Get a custom gauge, instead of MPH/kph, get one that's for 3.909/4.778

Id like the convenience of the nice analog needle gage in my stock gage cluster.
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Old 12-17-2014, 05:00 PM   #6
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Id like the convenience of the nice analog needle gage in my stock gage cluster.
If I'm not mistaken, what Curly is suggesting is instead of calibrating the needle to point to the correct number on the existing gauge face, just get a new face for the existing gauge that is calibrated to the needle. You'd still have an analog gauge.
Have it made with two sets of markings, and instead of those markings being MPH/KPM it's for your two different rear end gear setups.
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Old 12-17-2014, 05:05 PM   #7
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Originally Posted by Boost_creep View Post
If I'm not mistaken, what Curly is suggesting is instead of calibrating the needle to point to the correct number on the existing gauge face, just get a new face for the existing gauge that is calibrated to the needle. You'd still have an analog gauge.
Have it made with two sets of markings, and instead of those markings being MPH/KPM it's for your two different rear end gear setups.
This was going to be my route when if i ever get my 3.3 gear.

I'm sure it won't be SPOT on, but it will get me pretty dang close.

I'd just use GPS to calculate and note that when the speedo is in the X'o-clock position i'm doing Y speed. Do that for as many different speeds as possible. Then have him print the speeds in those locations.

30 minutes to swap out gauge faces. Even quicker than changing a speedo gear and less messy.
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Old 12-17-2014, 05:15 PM   #8
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I was going to suggest the same thing, you could probably change an instrument cluster faster than the speedo gear. Just have two, with one being an offset speedo.

The mileage and trip counter would be wrong, though.
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Old 12-17-2014, 05:21 PM   #9
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If he's going through the trouble of changing the rear end for different applications, a cluster should be easy.

Maybe make some type of quick connect plug ends for it and a snap in style system made out of something a little more robust than the OEM plastic.
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Old 12-17-2014, 06:12 PM   #10
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Originally Posted by Erat View Post
If he's going through the trouble of changing the rear end for different applications, a cluster should be easy.

Maybe make some type of quick connect plug ends for it and a snap in style system made out of something a little more robust than the OEM plastic.
the speedo gear swaps real easy way easer than the gage cluster like less than a minute to change. It takes about an hour to change the rear diff carrier assembly.
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Old 12-17-2014, 06:27 PM   #11
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Interesting it looks like all the rx7 stuff has left hand helix as well and the Miata is right hand. speedo would go backwards. I dont know how to determine which models can interchange parts.

86-92 RX7 convertable vs. 4:30 miata. note the teath slant the opposite direction.
Attached Thumbnails
A different Speedometer gear question?-0862-17-441adetail.jpg   A different Speedometer gear question?-speedo-drive-gear-mazda-mx-5-mk1-4_3-diff-23-teeth-5249-p.jpg  

Last edited by bbundy; 12-17-2014 at 06:49 PM.
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Old 12-17-2014, 06:29 PM   #12
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Doesn't some kind of electronic box exist where you can just tell it to input one signal, modify it, and output another? I swear I've seen someone reference one somewhere. That would make things easy.
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Old 12-17-2014, 06:31 PM   #13
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That doesn't work with mechanical speedometers.
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Old 12-17-2014, 08:57 PM   #14
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Quote:
Originally Posted by bbundy View Post
Interesting it looks like all the rx7 stuff has left hand helix as well and the Miata is right hand. speedo would go backwards. I dont know how to determine which models can interchange parts.

86-92 RX7 convertable vs. 4:30 miata. note the teath slant the opposite direction.
That gear is just pressed on and plastic. Try and find some hobby gear or something that will press on that shaft that works for you.
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Old 12-18-2014, 05:21 PM   #15
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I think I have it figured out if in fact the M501-17-341 is a 7t drive gear Im looking for. Its a good part number on Mazdamotorsports.

driven gears sleve housing
17T M501-17-441 1669-17-442B 89 B2000
18T M508-17-441 1011-17-442A 86 929
19T M502-17-441 1011-17-442A 86 929
20T M503-17-441 1011-17-442A 929
21T M504-17-441 1011-17-442A E2000
22T M510-17-441 1669-17-442B NA miata with 4:10
23T 1472-17-441A 1669-17-442B 90-92 NA miata with 4:30


Drive Gears
6T M503-17-341A NA-NB Miata 5 or 6 speed
7T? M501-17-341A 1988 Mazda E2000 with 4.778

swapping to a 7T drive gear in the transmission would make it so I could get what I need with a quick dreven gear swap.
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Old 12-18-2014, 05:42 PM   #16
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Quote:
Originally Posted by curly View Post
Use math or GPS to figure out your actual speeds.

Email RevLimiter

Get a custom gauge, instead of MPH/kph, get one that's for 3.909/4.778
I have always admired intelligent people who are able to offer practical solutions.
Props.
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Old 12-18-2014, 05:47 PM   #17
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The following post is theory, but its something I've considered:
  1. Convert to NB electronic sender
  2. Buy YellowBox
  3. ???
  4. Profit

The yellow box should allow you to correct for the ratio changes. I believe the NA still has an electric pulse generator thing on the end of the cable in the back of the cluster so its possible you could use the yellow box to hijack that signal and do the correction rather than moving to the NB sender. Food for thought anyway.
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Old 12-18-2014, 06:58 PM   #18
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Quote:
Originally Posted by EO2K View Post
The following post is theory, but its something I've considered:
  1. Convert to NB electronic sender
  2. Buy YellowBox
  3. ???
  4. Profit

The yellow box should allow you to correct for the ratio changes. I believe the NA still has an electric pulse generator thing on the end of the cable in the back of the cluster so its possible you could use the yellow box to hijack that signal and do the correction rather than moving to the NB sender. Food for thought anyway.
Profit?

Seems way more expensive and time consuming than a $13 gear that takes less than a minute to swap out with one tiny bolt while the rear carrier gets swaped.
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Old 12-18-2014, 07:25 PM   #19
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Quote:
Originally Posted by bbundy View Post
Profit?
Sorry, Its from a cartoon and I watch too much TV.
Quote:
Originally Posted by bbundy View Post
Seems way more expensive and time consuming than a $13 gear that takes less than a minute to swap out with one tiny bolt while the rear carrier gets swaped.
True. I figured it might be another avenue to explore, depending on how things turn out. Its something I stumbled across while trying to not buy the $120 sender for my 3.63 diff.
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Old 12-19-2014, 02:52 PM   #20
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Does the NA mechanical spedometer read positive speed in reverse?

Wondering if I can go to gears that spin it the opposite direction and have it work. It seems to be easyer to find those parts.
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