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Discussion Starter · #1 ·
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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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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Discussion Starter · #4 · (Edited)
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.
 

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Discussion Starter · #5 ·
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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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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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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Discussion Starter · #8 ·
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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Smiths Speedos: 1000 type and 1600 type, and gearboxes

The "1000" (150 MPH speedo) or "1600" (120 MPH speedo) appearing on the speedo face indicates the number of revolutions per minute that the speedo cable turns at 60 MPH. The rear wheel (with original Dunlop K70 400x18 tyre) turns "about" 800 RPM at 60 MPH. The 15/12 type gearbox drives the cable at 1.25x, to suit a "1000" speedo. The alternate type (I don't recall the number of gear teeth) gearbox drives the cable at 2X, to suit a "1600" speedo.

If in doubt, one can insert a small screwdriver into the gearbox cable drive to indicate the number of turns per 1 wheel revolution.
 

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Discussion Starter · #12 ·
Thanks mark and blue and all others that contributed. I have the 150 MPH speedo so I should and do have the 15/12 gearbox, therefore if it is reading low, it has to be the speedo head that needs attention as I have the new gearbox and new speedo cable. I had read a post here on the forum that indicated that 70 MPH at 4,200 RPM was what they registered and thought (incorrectly?) it might hold true for all twins (mine was indicating 55 MPH at that RPM). As I noted in previous a post, my manual said the gearbox ratio should be 2/1 but I have found other "misinformation" in it so wasn't sure. Since I thought the speedo was reading lower than it actually was due to the 70 at 4200 info and the ratio wasn't what the book called for (two bits of erronious info), I thought mine was way off and started my "wild goose chase". I have since taken the speedo off and shot a small squirt of a silicon lube into the spigot where the cable goes in. The indicated speed came up a little and now it reads between 55 and 60 MPH at 4,000 RPM. The needle does wiggle a bit so I'm thinking the head probably needs a going over by a pro. I'll probably contact the people GPZ recommended few posts earlier. (thanks GPZ, and you sent the right part, sorry about the mess)
 
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