Speedo gear box ratios. - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 12 (permalink) Old 04-18-2009, 10:29 PM Thread Starter
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Speedo gear box ratios.

I must have gotten the wrong speedometer drive gear box. I put a new one on when I started the rebuild on my 68 T120 and after comparing what speed was registering to my 2006 T-100 ridden by my brother, I've have found the speedo reads 8 to 10 mph slow at 55 to 60 mph. Sh!!!
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post #2 of 12 (permalink) Old 04-19-2009, 01:34 AM
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What is the ratio of gearbox you used also what are the numbers on the face of your speedo. Ever think your speedo is correct and your brothers is off?
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post #3 of 12 (permalink) Old 04-19-2009, 06:03 AM
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Speedo's out

Do either of you have a satnav that give a speed reading?
Also, both your speedos may be out by 10%...

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post #4 of 12 (permalink) Old 04-19-2009, 10:12 AM Thread Starter
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The numbers on the speedometer are SSM 5001/06 1000. I dont know what ratio the gearbox is. When I ordered it I was just learning about Triumph and I ordered it from a guy that restores British bikes. I told him I needed the gear box for a '68' T120R.
I know all the motorcycles have a bit of generous error but I'm pretty sure the 68 is the one with the biggets error and it is a negative reading. On the freeway, I keep up with traffic and it is registering about 55mph. No way is that what reality is around here. I might believe it if it was reading 75 or 80mph.

Last edited by jimmy bush; 04-19-2009 at 10:14 AM. Reason: add info.
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post #5 of 12 (permalink) Old 06-19-2009, 02:39 PM Thread Starter
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I pulled the "new" gearbox off and it has Ratio 15/12 stamped on the inner ring. The stripped gearbox that was on the bike when I bought it was a 15/12 but that doesn't necessarly mean it is the correct one. My shop manual calls for a 2/1 ratio in the general data section. The speedometer head is the correct one as per the parts manual. So does any body know how fast a '68' T120 should be going at 4,000 RPM?
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post #6 of 12 (permalink) Old 06-19-2009, 02:58 PM
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If you have the wrong ratio box, it will read about 30 when going 70, or 70 when going 30, depending on the wrong combo.

10% error is very typical, it gets worse as the meters deteriorate.

It can be dangerous on a tach, you think you'r spinning 5,000 and you're spining past red-line.

www.joellevinecompany.com is an excellent rebuilder.

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post #7 of 12 (permalink) Old 06-19-2009, 03:04 PM
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Jimmy, I'm trying to recall if you supplied me with the part number, or if I researched it, but my '68 parts book shows D637 (60-0637) which is what you received. Was it a Smiths or an aftermarket unit?

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post #8 of 12 (permalink) Old 06-19-2009, 05:43 PM Thread Starter
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It was an after market from JRC. The box was marked BG5330/168. If it would be off as much as you say, it must be my speedo head needing overhauled or lubed.
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post #9 of 12 (permalink) Old 06-19-2009, 07:06 PM
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I've sure not heard of the aftermarket speedo drives OR aftermarket Smiths replica speedos being off by even 10%...

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post #10 of 12 (permalink) Old 06-20-2009, 07:26 AM
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Rpm

Jimmy,
4000 rpm is about 60 mph.
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