speedometer question

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ksd:
Having just installed an overdrive in my TR4 that was originaly built  without overdrive I'm wondering what effect this might have on my speedometer indication.
Am I indicating high, low, or the same? Thanks:  K.S.D.

TR3driver:
Should have no effect at all, unless the new overdrive has the wrong speedometer gear in it.  The takeoff point for the speedo is after the overdrive.

lfew1:
By coincidence, I have a related question.  My TR4A also acquired an OD at some point, though everything else in the drivetrain is original.  My speedometer reads 10 MPH too high pretty much throughout the speed range.  My tires and wheels are stock size (probably slightly smaller overall than the original radials which had I think an 82 aspect ratio rather than the 80 now available) but not enough to make a 10 MPH difference.  Could the gear be the culprit?

Thanks,

LFW

TR3driver:
If the error is constant across the range, then the problem has to be the speedo head.  A problem with the drive gears can only introduce a proportional error.  You can also check the odometer meter accuracy; a problem with the gears will also make the odometer inaccurate (by the same amount) while generally speedo problems will leave the odometer correct.

There is a good guide to correcting your speedo head at
http://home.comcast.net/~rhodes/speedo.pdf

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