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The biggest factor with the speedometer accuracy, besides their age now, =
is tire circumference.
=20
Looking just at the math for this from the speedometer to the tire using =
stock components.
=20
MK I 980 rev/mi speedometer =E2=80=93 15 tooth gear is totally =
accurate with a tire circumference of 74.48=E2=80=9D
980 speedometer =E2=80=93 16 tooth gear =E2=80=93 =
69.82=E2=80=9D
I have always been told that the 16 tooth works better with todays =
radial tire size.
=20
MK II 1020 speedometer =E2=80=93 17 tooth gear =E2=80=93 =
73.67=E2=80=9D
=20
This is the sweet spot =E2=80=93 tires need to be slightly bigger so =
they wear through that circumference.
Change differential gears and it all changes again.
=20
Ron Fraser
=20
From: Tigers <tigers-bounces@autox.team.net> On Behalf Of Will Seay
Sent: Monday, June 07, 2021 12:02 PM
To: Larry Mayfield <drmayf@mayfco.com>
Cc: Tiger Autox <TIGERS@autox.team.net>
Subject: Re: [Tigers] 15 tooth
=20
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.
=20
Will - 382001570=20
=20
=20
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vlink=3Dpurple><div class=3DWordSection1><p class=3DMsoNormal>The =
biggest factor with the speedometer accuracy, besides their age now, is =
tire circumference.<o:p></o:p></p><p =
class=3DMsoNormal><o:p> </o:p></p><p class=3DMsoNormal>Looking just =
at the math for this from the speedometer to the tire using stock =
components.<o:p></o:p></p><p class=3DMsoNormal><o:p> </o:p></p><p =
class=3DMsoNormal>MK I=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 980 =
rev/mi=C2=A0 speedometer =E2=80=93 15 tooth gear is totally accurate =
with a tire circumference of 74.48=E2=80=9D<o:p></o:p></p><p =
class=3DMsoNormal>=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 980 speedometer =E2=80=93 16 tooth =
gear =E2=80=93 69.82=E2=80=9D<o:p></o:p></p><p class=3DMsoNormal>I have =
always been told that the 16 tooth works better with todays radial tire =
size.<o:p></o:p></p><p class=3DMsoNormal><o:p> </o:p></p><p =
class=3DMsoNormal>MK II =C2=A0=C2=A0=C2=A0=C2=A0 1020 speedometer =
=E2=80=93 17 tooth gear =E2=80=93 73.67=E2=80=9D<o:p></o:p></p><p =
class=3DMsoNormal><o:p> </o:p></p><p class=3DMsoNormal>This is the =
sweet spot =E2=80=93 tires need to be slightly bigger so they wear =
through that circumference.<o:p></o:p></p><p class=3DMsoNormal>Change =
differential gears and it all changes again.<o:p></o:p></p><p =
class=3DMsoNormal><o:p> </o:p></p><p class=3DMsoNormal>Ron =
Fraser<o:p></o:p></p><p class=3DMsoNormal><o:p> </o:p></p><p =
class=3DMsoNormal><b>From:</b> Tigers =
<tigers-bounces@autox.team.net> <b>On Behalf Of </b>Will =
Seay<br><b>Sent:</b> Monday, June 07, 2021 12:02 PM<br><b>To:</b> Larry =
Mayfield <drmayf@mayfco.com><br><b>Cc:</b> Tiger Autox =
<TIGERS@autox.team.net><br><b>Subject:</b> Re: [Tigers] 15 =
tooth<o:p></o:p></p><p =
class=3DMsoNormal><o:p> </o:p></p><div><div><p =
class=3DMsoNormal><span style=3D'font-size:12.0pt'>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.<o:p></o:p></span></p></div><div><p class=3DMsoNormal><span =
style=3D'font-size:12.0pt'><o:p> </o:p></span></p></div><div><div><d=
iv><p class=3DMsoNormal>Will - 382001570 <br><a =
href=3D"mailto:Tigers@embarqmail.com"; =
target=3D"_blank">Tigers@embarqmail.com</a><o:p></o:p></p></div></div></d=
iv><p class=3DMsoNormal><o:p> </o:p></p></div><p =
class=3DMsoNormal><o:p> </o:p></p></div></body></html>
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