[Healeys] Incorrect Speedo Reading

[Eric (Rick) Wilkins]

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.