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Re: [Healeys] Incorrect Speedo Reading

To: Bruce Griffin <bruce_griffin@earthlink.net>
Subject: Re: [Healeys] Incorrect Speedo Reading
From: Charlie Baldwin <mgcharlie@comcast.net>
Date: Fri, 28 May 2010 08:19:53 -0400
Bruce,
Your question is an interesting one, but I'm afraid there isn't an easy 
answer.
In comparing the reading on my BT7's speedometer to the GPS readout, I 
have found that around 50mph the two instruments are in agreement.  As 
the speed gets below that, the speedo reads fast meaning that I'm 
actually going slower than indicated.  At speeds above 50 the car's 
speedo starts to fall behind so that my actual speed is greater than the 
speedometer says it is.
This all makes sense to me since there isn't a mechanical coupling of 
the speedo cable and the indicator needle on the Smiths speedometer.  
The coupling is by a magnet which could explain the phenomenon that I 
have observed.
Also, after all of the time since the cars were manufactured, they could 
vary greatly from one speedo to another, depending on how much magnetism 
they have retained.
And BTW, I am running a Toyota transmission, which probably has a 
speedometer drive gear different than stock.  I didn't have a GPS when 
the stock transmission was in the car, so I can't speak to that.
I also have a stock BT7, but have not checked that against a GPS.  That 
car has only 14,000 miles on it, so there would be little wear in the 
system, but time could have degraded the magnet.
Of course your car could be totally different.
Charlie

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.
>
> This is also a good math problem for my 11yo son.
>
> Regards,
> Bruce D. Griffin
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