Model 18 Speedometer Gears

I haven't seen a thread on a list of gears. I'll post up what I've found and hope people can help clean it up and make it useful. I do have a question regarding all the different tire and wheel sizes available. It seems to me there should have been more speedometer gear options for all the sizes discussed here Original OEM Tire Sizes on Early CJ5s

For example - given a CJ with the F4 - some options were 6.85x15, 7.00x15, 6.00x16 which yield slightly different overall wheel height. If they were to all go on the same axel ratio wouldn't the speedometer reading be different for each?

 

Here is the best I've seen so far for a breakdown. Please chime in and post a better one or help make this better if there are errors.

DANA 18 SPEEDOMETER GEAR CHART

JEEP part #.....SW part #......teeth.....final drive ratio.....years.......models

A-1511.............447021............4.............Drive Gear...........41-71.......MB / GPW / M38 / M38A1 /
A-5707.............448110............15.............4.88..................41-45........MB / GPW /
A-5707.............448110............15.............5.38..................45- 71.......M38 / M38A1 /

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A-1511.............447021............4.............Drive Gear...........45-62........CJ / with 134 engines
A-5707.............448110............15.............5.38..................45-62........CJ / with 134 engines

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642830............447096............4.............Drive Gear...........47-57........SW / PU / FC / with 134 engines
642830............447096............4.............Drive Gear...........54-57........SW / PU / FC / with 226 engines
643710............xxxxxx.............13.............4.88...................54-57........SW / PU / FC / with 226 engines
644118............448162............15.............5.38...................47-57........SW / PU / FC /with 134 engines
644421............xxxxx...............xx..............xxx.....................xx-xx.........xxxxxxxxx

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910086.............447331...........6............Drive Gear..........58-71..........CJ / SW / PU / FC / with all engines
xxxxxx..............448361...........15.............3.73 ?...............xx-xx..........xxxxxxxxxxx
910087.............448493...........18.............4.27.................62-??.........CJ with 134 engine
910088.............448494...........20.............4.88.................58-62.........SW / PU / FC with I-6 226
910088.............448494...........20.............4.88.................66-??.........CJ with 225 engine
910089.............448495...........22.............5.38.................66-71.........CJ with 134 engine

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931381.............447537............6............Drive Gear..........6?-71.........CJ with 134 + 225 engine ?
931381.............447537............6............Drive Gear..........63-64.........SW / PU with 230 engine
931382.............448855............15.............3.53.................63-64.........SW / PU with 230 engine.
931383.............448856............16.............4.09.................63-64.........SW / PU with 230 engine
931384.............448857............17.............4.27.................63-64.........SW / PU / with 230 engine
931385.............448858............18.............4.89.................63-64.........SW / PU / with 230 engine
931384.............448857............17.............4.27.................6?-71.........CJ / with 134 engine
931385.............448858............18.............4.89.................6?-71.........CJ / with 225 engine

 

That chart looks like something I posted.

Yes different tire diameters do effect speedometer accuracy.
For that reason the speedometer gears will only get you in the ball park concerning accuracy.

Example:
Going from 4.88 to 5.38 is roughly a 10% change
But going from standard 6.00 x 16 tire at 28.5" tall to a 6.50 x 16" tire at 30" tall is only a 5% change.

What combination of gearing and tires do you have ?

 

I have 4:27 gears and 7.00 x 15 tires.

 

These would be the factory gears for 1963 CJ with 4.27 ratio.

910086.............447331...........6............Drive Gear..........58-71..........CJ / SW / PU / FC / with all engines

910087.............448493...........18.............4.27.................62-??.........CJ with 134 engine (driven gear)

 

Yes, that's what I believe is correct for me. But what if I was using 6x16 tires. Both were options right? The difference would be significant enough to warrant different gears would it not? How did the manufacture deal with this given the different wheel and tire options throughout the years?

 

Standard tire from 1945 CJ through 1962 CJ was the 6.00 x 16" at roughly 28-1/2" tall.
Those Jeeps were never dead on accurate concerning the speedometer calibration.
Those particular speedometer gear sets are actually more accurate when running 6.50 x 16" tires at nearly 30" tall.
A-1511.............447021............4.............Drive Gear...........45-62........CJ / with 134 engines
A-5707.............448110............15.............5.38..................45-62........CJ / with 134 engines

But you have a later post 1962 speedometer gear set.

Tire diameters and widths certainly can and do vary from one manufacturer to another.
For example STA tires tend to run taller and wider than most other brands for a given size.
That said the differences are fairly insignificant concerning effects upon speedometer accuracy..
Your actual tire diameter installed can be compared to the standard factory tire for your Jeep.
So what is your actual inflated tire diameter ?

Yes in later years after 1962 Jeep did use certain speedometer gear sets for specific tires at various differential gear ratios.
That information is listed in the various parts catalogs.
But it certainly does not include all possible combinations of ratios and diameters.
That's why I suggest that the ratio vs diameter need = percentage of change.

Getting the speedometer to read accurate is a bit of trial and error.
The best I could do is to check on the standard 1963 tire diameter and estimate deviation from standard diameter.

 

I created a post years ago here that helped with these tooth counts and matched sets.
Probably lost somewhere in the archives.

 

Looks to me like the 6.70 x 15 tire and the 6.00 x 16 tires are virtually the same diameters .
Both roughly at 28.5 inch tall.
So there should be little concern with regard to speedometer gear sets.

I suggest you install the 447331 drive gear with the standard 448493 driven gear.

For any given engine velocity (RPM):
A taller tire will increase travel speed or decrease the speedometer reading.
A lower differential ratio (numberically higher) will decrease the travel speed or increase speedometer reading.
Adding teeth to the driven gear will confuse even the experts.... Ha Ha Ha !
It's just like opening a can of worm gears Ha Ha !

So if your speedometer is reading high or low you can try a different driven gear.

 

My inflated tire size is 29.5. I'm going to hopefully take it for a drive tomorrow after replacing the gears. I'll updated the group.

 

My speedometer is about 1 mph slow if at all now with the 6 tooth drive and 18 tooth driven. Pretty good!