>From: idi!berry@apple.com (Berry Kercheval)
>In-Reply-To: Robert Jones's message of Thu, 7 Feb 91 11:30:54 EST
><9102071630.AA07875@elroy.meaddat>a.com>
>
> My last question concerns the speedometer. It reads high, by at least
>15-20%
> although I haven't figured out exactly how much high it is. I mentioned
> that I thought it was high to the guy I bought it from and he said it was
> because it had bigger than stock tires. This doesn't seem right, it should
> read slower, shouldn't it? What can be done to correct a speedometer? Is
> replacement in order? Could it be the cable?
>
>I think speedo shops can change out the speedo drive gears in the
>tranny, or calibrate the instrument itself. I have the same problem
>on my TR-6. Cable is unlikely, 1 rotation in=1rotation out of the
>cable. Bigger tires should make it read slower, yes, but remember
>that if a previous owner installed Webers you may also have a
>non-standard rear-end gear ratio. ( I *think* the MGB speedo drive is
>on the tail of the gearbox; if it's on a wheel disregard all this.)
Another reason for high speed readings is a long term weakening of
the speedometer needle return spring. One way to verify if the
correct gearing (speedo, rear, etc) is in place is to ceheck out
the odometer accuracy. Does it read high also? If the odometer
gives a reading that is consistent with the actual miles driven,
then any speed reading inaccuracy is internal to the speedometer
(return spring, magnet allignment, etc)
The speedometer and odometer are driven off the same input cable,
but the odometer is a direct gear driven mechanism, while the
speed reading can be adjusted. The speedometer needle is
moved by a circular metal plate which is directly rotated by
the speedometer cable. The speedometer needle has one or two
magnets on a mechanism that is close to, but does not touch
the rotating plate. If the magnets are adjusted closer to
the rotating plate, the speedometer needle will be moved
to a higher speed reading for the same rpm of the rotating
disc. Move the magnets farther away and the readings will
be lower. The return spring also is involved, since if the spring
weakens, the needle will be easier to move.
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Bill Sohl || email
Bellcore, Morristown, NJ || UUCP bcr!taichi!whs70
(Bell Communications Research) || or
201-829-2879 Weekdays || Internet whs70@taichi.cc.bellcore.com
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