I don't have your math answer, but I can tell you that almost all
modern tires have a smaller rolling diameter than factory tires. That
will cause a speedo to read higher than actual speed.
A bouncing speedo can be fixed by cleaning and lubricating your
existing cable and cable angle drive.
Wilko
San Diego
>
>
> Bruce Griffin wrote:
>> Gentlemen,
>>
>> This a very basic question, but I am hoping for some insight to see
>> if there
>> is a common Healey reason for this.
>>
>> I know my speedo has been bouncing, so I want to replace the cable
>> to see if
>> that helps. My main question is how to estimate my speed based on my
>> obviously incorrect speedo reading. I checked my speed with another
>> car and
>> knew it was off. Today, I passed a PD temp. speed camera that
>> displays your
>> speed. It indicated I was going 40mph, while my speedo showed my
>> speed at
>> 55mph. If I remember my engineering training from college, this is
>> related to
>> the gear in the transmission and I should be able to adjust my
>> speed by using
>> the fraction 40/55. This assumes a portion for correction as
>> opposed to a
>> simple add 15mph because the gearing of the speedo drive gear is
>> adjusted by a
>> percentage, not a fixed number. Any advice appreciated.
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