At 12:21 PM +1100 10/3/99, Tony Rhodes wrote:
>I don't remember what model car you have. I presume TR6 from the CDnnnnn
>differential. I do not remember if the diff of the TR6's had CD, but I guess
>they did. Measuring the wheel and engine turns is the most reliable method.
>I have a hard time being sure about anything less than 1/4 revolution, so
>I can just barely be SURE of the difference between 4.1 and 3.7. It certainly
>SEEMS as if you have a lower ratio than 3.7. I am not sure of the options
>available for the TR6, but I believe there were lower ratios than the 3.7.
For greater accuracy, turn the rear wheel ten times and count the number of
crankshaft turns. For greater simplicity, chalk mark the drive shaft,
rather than the crank extension, and leave the car out of gear. If only
one rear wheel is being turned, I think the number of drive shaft turns
should double (ie divide the result by 20 to get the diff ratio).
The general consensus seems to be that carb TR6s had 3.7 and PI TR6s had
3.45. The Moss Europe catalogue shows 3.7 for TR250 and TR6 carb models to
~CC61570. It also lists 3.45 for TR6 PI models, but does not explain carb
models after ~CC61570. The implication is that later carb models had 3.45
diffs, but that is contrary to most other references.
The PI TR6 was more powerful, but it was also fitted with smaller tyres.
The 3.45 diff would theoretically be about 7 per cent faster than the 3.7,
but the 185 section tyres fitted to the carb model (compared with 165 on
the PI) reduces that difference to about 3.5 per cent in practice.
>Your calibration of 1120 says that you have 1120 speedo cable turns per mile.
>The speedo drive is 1/2.5 of the drive shaft rpm. So you have 2800 drive
>shaft
>turns per mile. Since I doubt that you have SMALLER tires than the TR4
>series,
>then your differential HAS to be a lower ratio than 3.7. It would appear to
>be, indeed around 3.5!
My carb TR6 has an 1120 speedo and it reads 60 mph at 2800 rpm (which only
proves that the calibration is about right). I have not checked the diff.
Trevor Jordan
74 TR6 CF29281U
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