> I keep seeing posts here and elsewhere that suggest that changing a
> standard gearbox to OD somehow affects the accuracy of the speedometer.
> Am I all wet or is the gearbox irrelevant in this equation?
Definitely not irrelevant. Gears inside the OD set how many times the
speedo cable turns per driveshaft revolution, which directly affects the
indicated speed. For early TRs, the speedo drive ratio was always the same,
but that was not true of all cars that used A-type ODs. So these days, with
people scrounging A-type ODs from all sorts of places (eg Volvo, Healey,
etc.), it's not uncommon to find a unit with something other than the
standard 5:2 speedo drive ratio in it.
For later TRs with J-types, the gears inside the OD were changed to match
the tires and final drive ratio; plus they were used on an even wider
variety of cars (like Spitfires with 13" wheels and 4.11 final drives).
This leads to even more chances for error.
> Mike -- There is certainly no need to look for tires based on what size
> will make your speedo more accurate. For the not-too-fussy these units
> can be adjusted to a fairly accurate speed indication by just moving the
> needle.
Moving the needle can only make it accurate at one speed.
If, for example, it currently reads low by 10% across the scale (as might be
caused by a ratio change), and you move the needle to make it accurate at 60
mph, it will now read 10% high at 30 mph, and 5% low at 90 mph.
And of course moving the needle has no effect on the odometer, so it will
still read 10% low.
> If more accuracy is needed or if a very accurate odometer is desired
> then a professional calibration &/or an adapter may be needed.
I have yet to find a professional willing to try to alter the original
calibration.
Randall
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