Speedometer ratio

I have a 2:1 speedo. It reads slow. The head has been check and is fine. Just interested in which way does the
ratio have to go to read correctly?

Your speedo is looking for 1600 revs per mile using a 2:1 gearbox on a wheel rotating 800 rev per mile but its reading slow so it needs a higher ratio gearbox say 21:10 or 2.1:1 fitted to some A10's and A65 plus others. You will need to sort out axle size variation's and what side of the wheel is supposed to fit when you find one. What you really need to do is recalibrate the speedo using the internal brass screw, if you drill a hole in the right spot you can get a screwdriver to the screw.

Onder said:

I have a 2:1 speedo. It reads slow.

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A standard Commando speedo?
How do you know it's "2:1"?
A Commando Smiths or Veglia magnetic speedo wouldn't usually have a ratio marked on it (rev counters do).

The standard drive gearbox to match all Commando speedos (regardless of MPH or k/mh) was 1.25:1.

If you are using a 1.25:1 drive gearbox with a "2:1"(?) speedo it would read slow.

kommando said:

Your speedo is looking for 1600 revs per mile using a 2:1 gearbox on a wheel rotating 800 rev per mile but its reading slow so it needs a higher ratio gearbox say 21:10 or 2.1:1 fitted to some A10's and A65 plus others. You will need to sort out axle size variation's and what side of the wheel is supposed to fit when you find one. What you really need to do is recalibrate the speedo using the internal brass screw, if you drill a hole in the right spot you can get a screwdriver to the screw.

Click to expand...

I did that internal adjustment on mine a few years ago. I picked up a cheapo bicycle lcd readout speedo with magnetic pickup, fitted to rear wheel and set its diameter into the bicycle speedo. With rear off ground, spun it up in gear to compare lcd readout to Smiths, and turns internal screw so they matched. Later verified on road running smartphone gps at various speeds.

Of course this adjuster only affects the needle spring resistance, and will not change the Odometer turning rate, though that doesn't need high accuracy generally (doesn't result in costly speeding tickets if wrong).

Somehow this thread was locked. I don't know how that happened. If I did it, then it was unintentional so I've unlocked it.

Onder said:

I have a 2:1 speedo. It reads slow. The head has been check and is fine. Just interested in which way does the
ratio have to go to read correctly?

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As I understand it, the face of the speedo should have 1000 or 1600 and this is the cable revolutions per mile. The rear wheel gearbox comes in only 15:12 (1.25:1) and 2:1 (if I remember right). Most Triumphs, Commandos, and a few BSA use 15:12. Most BSAs use 2:1. Of course, this relies on the rear wheel being the size it left the factory and the drive and head matching. There could be other speedo heads that I don't know about. All from memory so if not right, forgive me.

Not sure how you know the speedo is 2:1? Does it have a Norton face?

I have sinned. Mea culpa. Inadequate information on my part. My question was answered however. But since you all contributed let me start anew.
Police bikes got a chrono not a magnetic or at least some of them. I wanted to put a chrono on my 74. Got an old BSA chrono and sent it to a rebuilder. He told me it would need to be driven by a 2:1. Bought a new made gearbox. He also told me I would need a specific cable and he made me one. The speedo works but it reads slow. If I remember correctly (good luck) he said all chronos could be adjusted to was 2:1. In my box of old bits I have several
drive ratio smith gearboxes. Thought a different box might do. I shall have to enter The Cave again to see if I do indeed have a 2.1:1 box, or indeed if such even is a thing.
It would indeed be great if I could get this to work as chronos are interesting.

Onder said:

I shall have to enter The Cave again to see if I do indeed have a 2.1:1 box, or indeed if such even is a thing.

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Yes. BG5330/104, /106, /111, etc. (to suit various axle sizes).

Thx, I used to have a complete list of BG numbers but cannot seem to find it. Does someone have a good link to
same?

Onder said:

If I remember correctly (good luck) he said all chronos could be adjusted to was 2:1

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So they can be calibrated but the adjustment range will be limited but may be enough to read correctly. When I was pulled over by a motorcycle cop in the 80's he said his Speedo was calibrated daily. That was the reason plod used the chrono, accuracy and ability to be calibrated.

kommando said:

So they can be calibrated but the adjustment range will be limited but may be enough to read correctly. When I was pulled over by a motorcycle cop in the 80's he said his Speedo was calibrated daily. That was the reason plod used the chrono, accuracy and ability to be calibrated.

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I don't think they were actually calibrated just checked that they were accurate. Just outside the police headquarters in Worcester there used to be some posts at the side of the road that were a set distance apart (commonly known as the measured mile) and the idea was to do a constant speed between the markings while timing yourself. I could be talking nonsense of course but I was told that by a bike cop.