Speedo calibration and/or axle ratio change

[Michael D]

Silly question completely off topic: would such a Triumph remote gearshifter also work on a 3-Speed box or not?? Thinking on my Dutch wooden shoes I'd say it would, als long as the shifter properly engages with the box-mechanism? Or am I completely off my rockers?

[Robert Leigh]

Apart from my speedo change I have different intermediate ratios in the gearbox. Keith Rawson supplied a modified laygear cluster which has a new primary gear welded in place of the original and a different first motion gear. This has transformed the ratios to be far more useful, first being needed for a quick getaway and third taking me beyond 60 mph. The new gears are straight cut giving a whine which occasionally raises the comment 'I presume the whine is the supercharger'. My final drive ratio is 5.25. It is a combination which I find satisfactory.

[Parazine]

This is the same speedo problem that I encountered with my Chummy. The speedo is (what I call, for obvious reasons) an "A" type, fitted up to early 1927 and it was originally driven by the propshaft via a pulley system. It is geared for 3360 TPM.

The later 3 speed cars used a speedo directly driven from the gearbox, usually a Smiths "PA" type. This was geared for 2240 TPM.

The four speed gearbox drives a "Magmo" or a needle speedometer, these are geared for 1040 TPM (1060 for later types with a different axle ratio).

You will see that driving a speedo geared for 3360 TPM with gears delivering 1040 TPM will incur a reading of about 1/3 of the true speed.

Your solution?

Special gears were made at one time for the 4 speed box but these were aimed at driving a PA I think. Vince Leek will know more....

You could mount a later speedometer but it won't have "Vintage" looks.

You can use a "speed up" gearbox in line with a ratio of 3360/1040 = 3.23:1 but you might have to get someone to make it up specially. (££££)

I believe Willy McKenzie had some Smiths faces made that turn the "A" type into a 100 MPH speedo and put the figures in roughly the right place for you. Might be worth a call....

I use a three speed box on my Chummy, so I needed an "A" type speedo with 2240 TPM gearing so I built a "special", using a 2240 TPM governor from a PN speedo. It was a bit of a jiggle, but works well.

[Colin Cheesman]

Both the three speed gear and four speed are the same dimension internally, my solution is to machine a three speed gear to same width as the four speed, the only other modification is to machine the nipple off the bottom of the drive pinion from the three speed gearbox. Both gear and pinion drive will fit the four speed gearbox with three speed ratios.
Hope this makes sense.

   

[Parazine]

Colin,

Won't fix the OP's problem as his speedo was originally driven from the propshaft and has 3360 TPM gearing. He'll end up with 2/3 speed reading......

He could follow your advice but would then need to build a special "A" type, like I did with PN type governor or use a PA or other 2240 TPM speedo.