[Healeys] Speedometer gearing

[Bob Spidell]

Per,

AFAIK, all 6-cyl cars with O/D came with 3.91 differential ratios.  The non-O/D cars came with 3.54 rearends (else they 
would run very high RPMs at highway speeds).  The 3.54 is a popular retrofit for the O/D cars to make them cruise even 
easier (I have one in my BJ8).

Look closely at the face of your speedo--you should see a little '1000' on the right below the needle.  That means the 
speedo is geared to indicate one mile of travel for 1000 revolutions (I don't know where you got 980, but you were 
close).  Look at your friend's speedo--the number should be smaller since you'll travel farther with each revolution of 
the driveshaft (a rough calc yields 905 or so).  1575 is way off--how did you arrive at that number?

The calibration for MPH is done by moving the needle (there is also a tiny brass screw on the support 'collar' for the 
needle shaft that seems to affect calibration, but it's mostly a mystery to me).

Bob

On 5/15/2011 1:25 PM, Per Schoerner wrote:
> Hi
> I'm trying to figure out the speedometer drive gearing inside the different gearboxes. I'm helping a friend with his 
> speedometer. He has a 100-6 which was delivered without overdrive. My own car is a 3000 mk2 with overdrive. The rear 
> axle gear ratio on these cars are the same, as far as I understand, according to Clausagers book. But the speedometers 
> are different. The 3000 speedometer needs 980 revolutions per minute to show 60 (MPH) and advance the odometer 1 mile 
> in one minute. The 100-6 speedometer needs 1575 revolutions per minute to do the same thing. Note that both cars are 
> imported from the states, so both are speedos are in miles.
> Does anyone know what the difference is? Well, I can calculate it of course, but I thought asking on the list would be 
> safe in case I have got something wrong.
>
> Best regards, Per from Sweden.
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Bob Spidell           San Jose, CA            bspidell at comcast.net

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