18 replies to this topic

[aus_yzman]

codes.oldholden.com/Speedo/

 

Easy to use speedo gear calculator

Edited by aus_yzman, 10 March 2015 - 05:21 PM.

[Bigfella237]

The link Mika posted is http://codes.oldholden.com/Speedo/

 

You need to know what colour the driven gear is (inside the extension housing), I'm guessing yours might be red if your speedo is that far out?

 

You will also need the size of the rear tyres (written on the tyre itself), it will look something like "175/75R13"

 

Once you know the colours of both gears and the tyre size, just enter them into that calculator and try different driven gears until the speedo reads accurate.

 

Although I suspect you may have to pull the back off the gearbox and change the drive gear to get it spot on?

[S pack]

Hi all,

 

I've got a dark blue 22 driven gear for my M20 4 speed gearbox, but the speedo is reading 100 kph when my car is doing 80 kph, obviously in mph, but you know what I mean.

 

Its in a LC Torana with a 186 and 3:08 diff ratio.

 

Does anybody know what colour I should use. I went onto the rarespares site, but its not telling me what I am seeing with the dark blue. So I cant trust the rarespares caculator.

 

I didnt know what tyre aspect ratio was. I put in .50 thinking it meant profile size?

22 tooth (Dk Blue) was original fitment drive gear for an LC Torana with 3.08:1 ratio diff.

If the drive gear (on the main shaft) in the M20 extension housing is white then you have (in theory) the correct drive gear & driven gear combo for your Torrie.

 

Your choice of tyres may be the culprit, or part thereof, however I suspect for the speedo to be reading approx 20kph above the true road speed then it would be prudent to entertain the possibility your speedometer is faulty.

 

Tyre aspect ratio is what is commonly referred to as the 'series' ie 50 series, 60 series etc etc.

 

Cheers

Dave.

[N/A-PWR]

Hello Glenn,

 

With 215/50/13 x M20 , white driver speedo gear x 3.08:1 Diff,

 

The calculator up there say's:-

 

- 104.8 kp/h using the Dark Blue.

 

- 100.1 kp/h using the Red.

 

So your speedo is reading wrong,

 

or you have a 3.55:1 diff which is 20 kp/h higher than actual speed using the dark blue

my bet is you need to check the 3.5 turns of the diff input = 3.55:1, if that is the case

per one turn of the wheels

[N/A-PWR]

Yep Glenn,

 

If both wheels turn one revolution, the ratio is closest to the yoke turns,

 

So if you turned 3 turns and the wheels did one turn each, then she is the 3.08:1 diff.

 

But if you are needing to go 3.5 turns to get the wheels to just do 1 turn each, then she is the 3.55:1 diff.

[ribz0]

I have used the calculator from above link... its close but you have play around a bit... I ended up using the next bigger gear from what it suggested and that was pretty much spot on...

[N/A-PWR]

Found this Speedo Gear calculator:

The link Mika posted is http://codes.oldholden.com/Speedo/

 

You need to know what colour the driven gear is (inside the extension housing), I'm guessing yours might be red if your speedo is that far out?

 

You will also need the size of the rear tyres (written on the tyre itself), it will look something like "175/75R13"

 

Once you know the colours of both gears and the tyre size, just enter them into that calculator and try different driven gears until the speedo reads accurate.

 

Although I suspect you may have to pull the back off the gearbox and change the drive gear to get it spot on?

Calculator = http://www.centauria...JcnMoGByzyOp-6Q

[claysummers]

I’ve used that calculator.
Funny thing, after replacing my speedo, it was reading about 10% fast compared to “GPS”. Prior to the failure I had it pretty well spot on within 3% at least. This is about the best you can expect due to the teeth increment being in the 4-5% range. I did a 200km run and compared the odometer against google maps however, and the odometer was within 2%. So I went up one tooth, to bring the speed within 5% and the odometer should be still within about 3%, albeit on the other side of correct.

Point of my story is that there can be some variation in old speedometers, and a good way to get yours reading accurately is to compare it to gps at speed, then vary the gear accordingly.


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[claysummers]

I need to get a light blue 25 tooth to complete my collection……


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[yel327]

I could have given you a box full. I gave away heaps of them, only kept the TH350 and TH400 ones. Most of the Aussie 4spds I had, the speedo cable was cut through and still had the end in the box.

[N/A-PWR]

Great idea in having a collection Clay,

 was my thoughts also,

 but to include the drivers too.

 

You can see which ones were used the most,

 or revved the most.

 

I’ve used that calculator.
Funny thing, after replacing my speedo, it was reading about 10% fast compared to “GPS”. Prior to the failure I had it pretty well spot on within 3% at least. This is about the best you can expect due to the teeth increment being in the 4-5% range. I did a 200km run and compared the odometer against google maps however, and the odometer was within 2%. So I went up one tooth, to bring the speed within 5% and the odometer should be still within about 3%, albeit on the other side of correct.

Point of my story is that there can be some variation in old speedometers, and a good way to get yours reading accurately is to compare it to gps at speed, then vary the gear accordingly.


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Updated the gears list:

 

Driven Gears

 

Pink 19 teeth

White 20 teeth

Green 21 tooth

Dark Blue 22 teeth

Red 23 teeth

Yellow 24 teeth

Light blue 25 teeth

 

Drive Gears

 

Red 7 teeth (rare)

White 8 teeth

Blue/Green 9 teeth

Yellow 10 teeth

 

 

I could have given you a box full. I gave away heaps of them

Been watching out for a bulk lot. 

[claysummers]

Never needed to change the worm drive fortunately Dave.


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[N/A-PWR]

I find it amazing how GM/Holden were able to maintain the driven gear outer tooth diameter,

 by making less teeth more coarse, & more teeth finer pitch,

 & yet the worm gear still drives any ratio in full mesh. 

 

Never needed to change the worm drive fortunately Dave.


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That calculator,

shows by changing the worm gear,

we can fine tune to within a 1/10th of a kp/h.