Speedometer gear?

My speedo was dead perfect with 3.91 gears. I swapped in 3.23s last night to make it a bit more cruiser friendly. Anyone know which speedo gear I need to put in now? Is there a calculator out there that helps figure it out?

So my best guess from what I have found is with a 28" tall tire and 3.23s I will need a 31 tooth green gear.

Laine

confirm what gear you have now & then do the math

in tech archive there is a speedo pinion chart click it then click on auto trans and type in tire size and gear ratio to get tooth count needed.

28 inch tire?

I used a 225/75/15 tire size = 28.2 x 3.23 = 30 tooth pinion

sound about right as my M/Ts are 29x15x15 = 325x50x15 = 28" tall and I run a 3.23 with 30 tooth seems dead on in mine.

if your 28" is on tall side them a 31 tooth is next.

google--dodge jeep speedo----enter the facts--including tire size and tranny gear ratios and you get everything....

Download a GPS based speed app on your phone, take the car out and compare indicated speed with the phone app at 30, 60 and 90 mph, do the math to calculate how much of a tooth decrease /increase you need.

Be sure to take 3 or more readings from slow to fast. and pick a range that is closest to where you drive most.

ie {@ 30mph indicated, GPS shows 25mph = 1.2

1.2 x what you have in there now = adjusted tooth count count.

For me, Freeway speeds can be hardest to judge consistently when the car has overdrive.

This one is a 3 speed manual trans. From what I can tell, either a 30 or a 31. Time to start pulling speedo gears out of all my transmissions that I have sitting around and see if I can find one.

Thanks for the help fellas.

Laine.

I use a 32 with 3.23 gears and 27.5" tires. I think the speedo may be slightly fast with this combo. something to keep in mind is the factories had the speedo's set a little fast and the feds forced them to provide accurate speedometers. I think the factory charts are some what off in favor of the factories and their warranties.

http://www.moparts.org/Tech/tech/img/AUTOMATICchart.xls

I found a 34 in a 4 speed I have laying around. Thats the closest Ive found to what I need. So far still no 31 or 32. More to check. ;-)

Laine.

The 32 is black (they are color coded also) I just installed one in a pals car.

32's are pretty common.

Found a 30 in one of my automatics. That will do for now.

Check ebay item 153042087853.

Are switching speedo gears the only reliable way to adjust an inaccurate speedo?

I used the calculator chart above to get an estimate of 27 teeth I should need with 225/70-14 tires and a 2.76 rear diff ratio..
I'm coming up with 6.2" sidewalls, plus 14" rims = 26.4" tire diam.
With a 2.76 diff, the result is 27 teeth for the speedo gear.. is that how it works?

The speedo in our '65 with cable-shift 727 automatic is indicating about 10% too fast (reads 90mph when actual is more like 80).
I'll pull the current gear and compare number of teeth in the car..

Thanks,
- Art

That chart is not accurate for the 65 style speedometer pinions.

Conversion to modern tire sizes

Snipped from the 65 Plymouth FSM

Looks like you need a 17 Orange pinion, assuming you don;t already have that. The coverage spread is kind of wide. Brewer's has used, $80.

Thank you,
That's a tremendous time saver - I have the FSM too and will have a close look.

Based on the tire sizes on that FSM chart, it looks like my speedo reading is high due to smaller tire size I've chosen. That would make sense because nothing else was changed from stock.

If I recall correctly, all the tranny oil will want to leak out when I pull that gear & cable coupling with the O-ring out of the tranny case... or maybe that was the shifter cable...
..either way I'll double check the FSM before I start..
...it's been 4 years since I finished the resto and I shouldn't trust my recall on such details..

Thanks again,
- Art

Jack up the drivers side and you can cut down on fluid loss.

The early 727 push button speedometer gears are not interchangeable with the 1966 and later 727 Way different in size

You could also have an inline ratio adapter put together similar to this
PICTURE LINK

they can speed up or slow down the speedometer. Google is your friend for a source. i have a local place that makes them up for us

#1 confirm the actual error with a GPS or interstate mile markers (go 5 miles/5 markers) for the odo & should go (1) mile in 1 minute with the speedo dead on 60 MPH for the speedo & usually when the speedo reads correct the odo will also but if not the speedo head would need to be sent to a speedometer specialty shop tho some people have successfully tweaked em at home. get the right speedo gear in there to correct the odo then see if the speedometer reads MPH correct. there's a bunch for sale on eBay & FABO, not cheap tho but it is important to get it dead on. EDIT meant to say that usually when the odo reads correct the speedo does also

Pretty much guarantee that adapter won't fit the early TF setup.

Notice that in the early setup the adaoter is part of the speedometer cable itself.

A 1964-65 B-BODY, 1963-65 CHRYSLER/IMPERIAL cable pictured, A body similar

Also a totally different design.

Interstate highway mile markers are a pretty reliable way to judge the offset. Sometimes state routes (2 lane black top) will also have them. The thing I did many moons ago: grab a car I knew read correctly, drive it about 10 miles, turn around. Note what it read out / back / combined. Take the exact same run in the one I knew didn't (tire, axle change) then calculate the offset. Put the tail on ramps to keep the fluid in the trans and swap gears then re-run the loop. As a final sanity check, I go looking for one of those road side construction zone things that flash your speed at you.

You can put an app on your phone that will tell you you speed continuously, they are pretty accurate and easier than any other method I seen listed.

They can split the cable and install the necessary fittings to place it anywhere between the cable ends

That gets beyond just buying a ratio adapter, particularly when it would be substantially cheaper and easier to just get the right pinion.

On a side note on the odo & speedo, if one of em is inaccurate I'd rather have the speedo correct. tickets ain't cheap!

Here's the thing with that: some of us were swapping gears before smart phones existed and old habits die hard.

True, but around here finding a mile marker is harder than finding gold at the end of a rainbow.

Here's what the early reduction gear looks like.

GEEZ!!! just offering the guy an option, it's his decision, not either one of ours