SS speedo gearbox ratio

Previously michael_pond wrote:

I need a new speedo for my 1964 650 SS. Thunderbird spares do a Replica one that looks correct and is a resonable price. They say it is suitable for a 15:12 or 1.25:1 gearbox. Can someone please tell me what Ratio my SS gearbox would be?

Cheers. Mick.

hello I do think in 1964 they changed the speed to a grey faced and these were 15:12 but the man to ask is Alan Gagg he dose speedo and rev-counters you find him in the link pages on this site.

My 1960 99 and 1967 Atlas are both 2:1 speed drives. The replica is not suitable with the standard 2:1 drive - though it does a fair approximation of kph if you do fit it. Even if you invest in a 15:12, it isn't particularly accurate, which is how I ended up forking out for a proper Smith's speedo and putting the nice but not terribly useful replica in a drawer.

All of the Dominators I have owned have used a 2:1 gearbox on the rear wheel. The Commandouses a 15:12 or 1.25:1 box. (Same number divided or mutliplied by 12.)

I am not certain what the Mercury 650 bikes used. They were around at the same time as theCommando so may have the same 15:12 speedo drive box. However, their speedo heads were the same as the Atlas. The Commando clocks were quite different.

I am sure that AJD would know the answers.

On Smiths magnetic MPH speedos if you look at the small numbers along the bottom of the face they normally end in 1600 or 1000, this is the number of turns per mile, as the rear wheel turns 800 per mile you fit the gearbox to suit the 1000 or 1600 as follows.

1000 goes with a 15/12 or 1.25:1 gearbox

1600 goes with 2:1 gearbox

There are exceptions of course and KPH is different as well

Previously john_holmes wrote:

On Smiths magnetic MPH speedos if you look at the small numbers along the bottom of the face they normally end in 1600 or 1000, this is the number of turns per mile, as the rear wheel turns 800 per mile you fit the gearbox to suit the 1000 or 1600 as follows.

1000 goes with a 15/12 or 1.25:1 gearbox

1600 goes with 2:1 gearbox

There are exceptions of course and KPH is different as well

I'm pretty sure the KPH are 600 & 1000 respectively (Turns per KM)

600 with a 15:12

1000 with a 2:1

Regards, Tim

Chronometrics also have the turns per mile on the face (1628 is a common one if I remember) - but beware - it's easy to change the face on a chronometricso the number on the face may not match the internal gearing.