[Healeys] Incorrect Speedo Reading

[Bruce Griffin]

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