[Tigers] 15 tooth

[Ron Fraser]

The biggest factor with the speedometer accuracy, besides their age now, is tire circumference.

 

Looking just at the math for this from the speedometer to the tire using stock components.

 

MK I       980 rev/mi  speedometer – 15 tooth gear is totally accurate with a tire circumference of 74.48”

                980 speedometer – 16 tooth gear – 69.82”

I have always been told that the 16 tooth works better with todays radial tire size.

 

MK II      1020 speedometer – 17 tooth gear – 73.67”

 

This is the sweet spot – tires need to be slightly bigger so they wear through that circumference.

Change differential gears and it all changes again.

 

Ron Fraser

 

From: Tigers <tigers-bounces at autox.team.net> On Behalf Of Will Seay
Sent: Monday, June 07, 2021 12:02 PM
To: Larry Mayfield <drmayf at mayfco.com>
Cc: Tiger Autox <TIGERS at autox.team.net>
Subject: Re: [Tigers] 15 tooth

 

Before you guys go off in search of the "correct" 15-tooth gear, you should consider other sources of error in your speed indication.  In stock form, the Smiths-Jaeger speedo is pretty linear, but it has an indicating error due to the fact that the indicating pointer is unbalanced.  This unbalance causes the speedo to read low (because it's lifting the weight of the pointer) for the first half of its rotation (0 - 70 MPH) and to read high for the second half (downhill from 70 MPH).  I balanced my pointer by coiling up a short length of solder and placing it in the base of the pointer to counter the weight of the pointer.  Just a rough balance is fine.  I then calibrated the speedo against GPS.  The calibration is done by just twisting the needle.  The needle is spring-loaded against the stop so the zero point stays fixed.   The speedo is now accurate to better that 1 MPH at up to 80 MPH.  That's with 185-70X13 tires and a 16-tooth gear.  Of course, the ultimate test of calibration is to check the odometer calibration.  I haven't done that, but you don't get speeding tickets for odometer errors.

 

Will - 382001570 
Tigers at embarqmail.com <mailto:Tigers at embarqmail.com> 

 

 

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