Speedometer Gear Reducer Adapter

[78gmcroyale]

Thanks JimWould you know how much decrease there is for each tooth added?Ra=
ySent from my U.S.Cellular=C2=A9 Smartphone -------- Original message --=
------From: Jim Kanomata via Gmclist Date: 11/7/2=
0 3:44 PM (GMT-06:00) To: gmclist Cc: Jim Kanomata Subject: Re: [GMCnet] Speedometer Gear Reducer Adapter OE=
M ones are light tan/white 31 teethOn Sat, Nov 7, 2020 at 7:40 AM 78gmcroya=
le via Gmclist wrote:> Quick question.....how man=
y teeth does the oem gear have?Sent from my> U.S.Cellular=C2=A9 Smartphone>=
-------- Original message --------From: Todd Snyder via Gmclist gmclist=
@list.gmcnet.org> Date: 11/5/20=C2=A0 3:52 PM=C2=A0 (GMT-06:00) To:> gmclis=
t Cc: Todd Snyder Subject:> Re: [GMC=
net] Speedometer Gear Reducer Adapter I don't think the mph reading> is the=
first thing to correct.=C2=A0=C2=A0 The first thing to get right is the> o=
dometer.=C2=A0 That's a direct geared readout ofdistance traveled.=C2=A0 It=
needs to> be correct before the speedometer reading can be addressed if ne=
eded.=C2=A0 You> could get on the highway, take note of the odometer as you=
pass a mile> marker.=C2=A0=C2=A0 Drive 10 miles and note the odometer read=
ing. Subtract onereading> from the other, figure out the % difference and g=
et a driven gear with that> % more or less teeth, depending on which way yo=
u need it to go.=C2=A0 Then pull> your current driven gear out and count th=
e teeth.=C2=A0 If the odometer is> reading low, you need fewer teeth on the=
driven gear.=C2=A0=C2=A0 If the odometer> reads high, you need more teeth.=
=C2=A0 You may or may not beable to get it> corrected with the driven gear =
though, the drive gear only works for a> certain range of tooth numbers. Lo=
oking online there aretransmission parts> stores that offer assistance with=
this.=C2=A0 They'll know what range of teeth> numbers will fit with what y=
ou have and sell you thegearThere are also> online calculators that can det=
ermine the correct driven gear to use but> you need to know the number of t=
eeth on the drive gear so theywon't be much> help unless you use them to fi=
gure out what's in there by trial and error,> which you can do. -- Todd Sny=
der, Buffalo NY 1976 Eleganza> II__________________________________________=
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________________________> GMCnet mailing list> Unsubscribe or Change List O=
ptions:> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org>--=
Jim KanomataApplied/GMC, Newark,CAjimk://www.gmc=
rvparts.com1-800-752-7502_______________________________________________GMC=
net mailing listUnsubscribe or Change List Options:http://list.gmcnet.org/m=
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[tmsnyder]

> I'm very confused by this statement.... "I don't think the mph reading is the first thing to correct.   The first thing to get right is the
> odometer.  That's a direct geared readout ofdistance traveled.  It needs to be correct before the speedometer reading can be addressed if
> needed."The speedometer cable feeds the speedometer and it's rate of spinning determines miles and mph.  It can't differentiate between the two. 
> If mph is correct then mileage should be correct and if mileage is correct then mph should be correct.  I don't understand how you can first fix
> one and then fix the other.What am I missing?

Hi Ray,

Yes that's how it _should_ work if your speedometer (only considering the mechanical unit in the dash) is calibrated correctly. After 40 years, I
would not make that assumption.

On mine, the odometer reads almost perfect b/c it is the OEM (31 tooth?) driven gear, OEM 3.07FD, and OEM diameter tires. Nothing has been been
changed. So as designed, the cable rotates 1000? turns per mile. That's a direct gear driven mechanism that turns the odometer. There's no
calibration other than getting the gearing ratios correct between the odometer and the output shaft of the transmission. Once those ratios are
chosen, it works until something physically breaks like the cable snaps or a tooth breaks off a gear. It's all or nothing.

There is no calibration drift in the odometer.

However, the speed reading part of the speedometer is another animal. I'm not an expert but to me it seems that the rotation of the speedo cable is
somehow coupled, maybe magnetically? to a spring loaded needle/pointer which gives, at best, an approximation of speed. That is a pretty fickle
readout which can fall out of calibration especially after 40+ years.

This is the reason that at one time there were places called 'Speedometer Shops' dedicated to calibrating this part of the speedometer. I doubt there
are many speedo shops still open for business. Maybe one or two in California if I had to bet.

On my GMC, there are parts of the speedo range where it is pretty much correct, and other parts where it is 10mph off. Now that we have GPS on our
cell phones, this is easy to observe. Back in the 70s no one would have known.

When I'm doing 70, the speedo says 60 but my odometer is recording miles spot on. The correct fix would be to have my speedometer calibrated at a
speedo shop, if one still exists.

The incorrect fix would be to change my speedo cable gearing to make the odometer read incorrectly but my speedometer read correctly. Does that make
sense? I've probably just muddled it up further, sorry.

Long story short: If you get the gearing right so that the odometer reads correctly, then your speedometer _should_ also read correctly. If it
doesn't, then the speedo could be sent off to a speedo shop for calibration.

--
Todd Snyder, Buffalo NY
1976 Eleganza II