Re: [AMC-list] speedo gears

[tom jennings <tomj@xxxxxxx>]

The speedo gear essentially matches the axle+tire size. It's entirely
possible for a car with and without OD to have the same speedo gear.

Here's the mental model to think about it: a 24" tall tire is 75" around.
Picture a white mark on the sidewall pointing straight down to the ground.
Roll the car forward exactly one tire revolution. You just went 75".  How
many 75" is there in a mile? (5280 feet * 12" = 63360"/mile) Tire turns
844.8 times in one mile. The speedo gear matches that.

But you don't even need to know that to fix the problem.

You can think of the driveline has having TWO transmissions -- one drives
the wheel, the other drives the speedo. It's got it's own gear ratio! That
is set by swapping gears to match the tire+axle combo.

What I've done to fix speedo errors is, calculate the error carefully, using
a measured mile. Say your odometer says you went 2.2 miles in a 2 mile
measured mile. Speedo reads high; 2.2/2.0= 1.1, or 110% or 10% too high. Big
gear (trans) driving a too small gear. You need a larger speedo gear.

So that's 10% too fast. Pull out the current gear, count the teeth. Say it's
17. 17 times 1.1 is 18.7. Ain't no such speedo gear; round to the nearest,
19. That's how many teeth you need.

The thing to keep in mind is, the gear cut into the transmission tailshaft
drives the plastic gear on the speedo cable. You need an "external standard"
eg. a measured mile. Picture those gears; if the speedo reads high, it's
spun too fast; the tailshaft gear speed is whatever it is at 60mph, but it's
spinning the speedo too fast. Change that gear (to a larger one) and the
speedo will slow down for the same tailshaft speed.




On Mon, Jun 1, 2009 at 23:07, <das24rules@xxxxxxxxx> wrote:

> ok, lets see if we can stump frank, or tom, or anyone else who knows the
> rambler goodies well. my speedo gear in the original NONE o/d trans was 16
> teeth. looked like a black, or really dark red fiber nylon or something.
> speedo gear for my o/d trans is the natural nylon color. i THINK it had 16
> teeth on it also. but the gear ratios are different for o/d and none o/d
> cars
> i thought. i am currently looking in the tsm at the speedo gear info
> section.
> none listed for 3.77 gear with 16 teeth. no color of red, but there is a
> black, and it has 14 teeth. 4.10 gear has 16 or 17 teeth, for the same tire
> size. i dont get the difference, but ok. o/d with 4.10 is also 16 and 17
> listed. those are with 15x6.50 tire size, 781 rev per mile. with 15x6.70
> 747
> rev per mile tires, still lists 14 teeth for 3.77 gears. 15 and 16 gears
> for
> 4.10 axle, o/d or not. so, this CONFUSES me. none of it makes sense to me.
> i
> would think different tire size, different speedo gear, different rear
> gear,
> different speedo gear. then sometimes, nothing changes but the speedo gear.
> i
> am trying to figure out what i need to do to make my speedo as close as
> possible. i dont THINK it should be off any from what it was before if the
> driven gear is the same on the o/d, and it shows that it is same as none
> o/d.
> i hope i confused someone as bad as i am... if both gears i have are the
> same,
> i will run the gear that goes to this trans, as my original one was looking
> rather worn. guess ill just have to see how bad its off, and take note.
> anyone know how to locate new gears if i need one??
> _______________________________________________
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>



-- 
All of your arguments are invalid.
Enjoy your unstable system.
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