Speedometer is driving me crazy- Something is Wrong

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AZ-Nick

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I am driving myself crazy trying to figure out what is wrong and I just can’t.
I purchased a 69 Swinger 340 auto trans a while ago with many small issues that needed to be addressed. One of the issues is the speedometer is way off, 12-13 mph off.
I know the gears have been changed but I don’t know to what.
I did all the calculations I could, rotating the tire and counting the revolutions of the driveshaft 10 times and it always came up more than 3-3/4 revolutions to a little less than 4 revolutions so I guessed 3:73 or 3:91 gears.
I also calculated RPM = MPH x Gear x Gear Ratio x 336 / Tire diameter and using a Department of Transportation Radar unit for the mph. This came out to 3:91.
Anyways I purchased 2 different speedometer gears for the 727 trans using the charts on multiple threads here.
The results are baffling.
Originally my speedometer was off between 13-14mph higher (43mph by radar and 56-57 on dash speedometer)
The original gear in the transmission was a 35 tooth.
I installed a 38 tooth and it corrected 7mph, I thought this is good so I installed the 40 tooth speedometer gear and it changed ZERO results, Nada, no changes, still off 7-8mph.
What am I doing wrong?
And before you ask me “Yes I know how to align the numbers with the teeth when installing the housing”
 
Last edited:
AZ-Nick said:
I am driving myself crazy trying to figure out what is wrong and I just can’t.
I purchased a 69 Swinger 340 auto trans a while ago with many small issues that needed to be addressed. One of the issues is the speedometer is way off, 12-13 mph off.
I know the gears have been changed but I don’t know to what.
I did all the calculations I could, rotating the tire and counting the revolutions of the driveshaft 10 times and it always came up more than 3-3/4 revolutions to a little less than 4 revolutions so I guessed 3:73 or 3:91 gears.
I also calculated RPM = MPH x Gear x Gear Ratio x 336 / Tire diameter and using a Department of Transportation Radar unit for the mph. This came out to 3:91.
Anyways I purchased 2 different speedometer gears for the 727 trans using the charts on multiple threads here.
The results are baffling.
Originally my speedometer was off between 13-14mph higher (43mph by radar and 56-57 on dash speedometer)
The original gear in the transmission was a 35 tooth.
I installed a 38 tooth and it corrected 7mph, I thought this is good so I installed the 40 tooth speedometer gear and it changed ZERO results, Nada, no changes, still off 7-8mph.
What am I doing wrong?
And before you ask me “Yes I know how to align the numbers with the teeth when installing the housing”
Click to expand...

One thing you may not have done is actually calibrate the speedometer itself.

I used these guys.

Great service, reasonable price, quick turnaround.


Welcome — Mr Heaterbox and Interior Restoration Services
 
You have a GPS in your phone Google maps displayed the speed.

Drive 60 mph on the phone, look at your speedo.

There is a possibility that the cable is rounded at the ends and is slipping.

Or the speedo is internally messed up.
 
Dana67Dart said:
You have a GPS in your phone Google maps displayed the speed.

Drive 60 mph on the phone, look at your speedo.

There is a possibility that the cable is rounded at the ends and is slipping.

Or the speedo is internally messed up.
Click to expand...
Great idea, this could rule out the Cable if it is bad or not. But on the other hand, if the end was rounded and slipping then the speed on the dash would be slower and not faster than the Radar posted speed wouldn’t it?
 
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AZ-Nick said:
Great idea, this could rule out the Cable if it is bad or not.
Click to expand...

Ahaha, sorry I just had to laugh at another Mopar speedo issue.
Been there and done that enough to say F it and just deal with it.

If I really need a speedo to tell me how fast I’m going I check the GPS speedometer on my phone.
40 is actually 35.
Ok then, send it.:D
 
May be cheaper/better to buy a good working speedo. Last I checked, gauges for an early A barracuda were very affordable, yours might be similar
 
AZ-Nick said:
Thanks, that’s one thing that I never thought of.
Click to expand...

I got in trouble with the wife one day when we were cruising her dart and the speedo let out a scream akin to a monkey high on meth.
She about changed lanes instantly.
Scared the crap out of her.
Yea, I laughed.
 
TrailBeast said:
I got in trouble with the wife one day when we were cruising her dart and the speedo let out a scream akin to a monkey high on meth.
She about changed lanes instantly.
Scared the crap out of her.
Yea, I laughed.
Click to expand...
That same Monkey has screamed at me before!
 
By my calculations

Dash 56 real 43

Going from 35 tooth to 38 brings your speedo down from 56 to 51mph

Going from 35 tooth to 40 brings your speedo down from 56 to 49 mph

You would need to be around 44 to get to 44 mph

And 46 gets it to 42 mph.

BUT PLEASE DO NOT TAKE MY WORD for the gear you need. at 25-50 each it is an expensive shotgunning

If you look in the factory service manual.

There is a tire size (you will have to find a conversion chart from letter / English number size to metric size conversion) rear gear ratio and number of tooth chart.

you should do a better job determining your rear ratio if it comes out to 3 3/4 then it is most likely 3.73. if it is closer to 4 then 3.91

the radar on the side of the road is far from accurate, so use your phone or a Garmin etc.

I don't think there is anything wrong with your speedo now you just did not go far enough.

What is the diameter of your rear tires?



1708325708885.png


1708326063683.png
 
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Dana67Dart said:
By my calculations

Dash 56 real 43

Going from 35 tooth to 38 brings your speedo down from 56 to 51mph

Going from 35 tooth to 40 brings your speedo down from 56 to 49 mph

You would need to be around 44 to get to 44 mph

And 46 gets it to 42 mph.

BUT PLEASE DO NOT TAKE MY WORD for the gear you need. at 25-50 each it is an expensive shotgunning

If you look in the factory service manual.

There is a tire size (you will have to find a conversion chart from letter / English number size to metric size conversion) rear gear ratio and number of tooth chart.

you should do a better job determining your rear ratio if it comes out to 3 3/4 then it is most likely 3.73. if it is closer to 4 then 3.91

the radar on the side of the road is far from accurate, so use your phone or a Garmin etc.

I don't think there is anything wrong with your speedo now you just did not go far enough.

What is the diameter of your rear tires?



View attachment 1716209052

View attachment 1716209053
Click to expand...
215/70x14 = Tires are 25.9” so I calculated 26 inches

IMG_4744.png
 
TrailBeast said:
I got in trouble with the wife one day when we were cruising her dart and the speedo let out a scream akin to a monkey high on meth.
She about changed lanes instantly.
Scared the crap out of her.
Yea, I laughed.
Click to expand...
I heard that same monkey in Bazza's '71 VG, and because of all of the work I had just done on it, it scared the hell outta me!

Bazza just laughed when I told him about this, and he said it's been that way for years.

I called him an a**hole.

He just laughed again!
 
ProjectBazza said:
I heard that same monkey in Bazza's '71 VG, and because of all of the work I had just done on it, it scared the hell outta me!

Bazza just laughed when I told him about this, and he said it's been that way for years.

I called him an a**hole.

He just laughed again!
Click to expand...

Now see?
I have heard a few variations of the screaming monkey, so I laughed when the wife’s car did it.
She called me an A hole.
 
Sorry to hijack but is there more than 1 worm gear for the 727's? Reason I ask is I got a 39 tooth (green) gear to swap out whatever was in it and when I pulled the original gear out, it was quite a bit smaller in diameter (don't know how many tooth) than the new gear. Because of this, I didn't think the gear would fit and sure enough, I could not install the new gear.

Thanks in advance. Bill
 
Texas speedo methods...seat of pantsfell fr speed and IF no one passes me, I must be doing right.
Or have wife drive the truck at 60 and I follow and check on tach. Then just go by tach.
I also use tack to tell me when low on gas!!
 
barbee6043 said:
Texas speedo methods...seat of pantsfell fr speed and IF no one passes me, I must be doing right.
Or have wife drive the truck at 60 and I follow and check on tach. Then just go by tach.
I also use tack to tell me when low on gas!!
Click to expand...
I got a ticket for 43 in a 40 once so I need it to be a little more accurate. Unfortunately living where I live I would need a clipboard and a sheet of paper with different rpm/mph on it for all the different speeds.
 
-Boosted- said:
Sorry to hijack but is there more than 1 worm gear for the 727's? Reason I ask is I got a 39 tooth (green) gear to swap out whatever was in it and when I pulled the original gear out, it was quite a bit smaller in diameter (don't know how many tooth) than the new gear. Because of this, I didn't think the gear would fit and sure enough, I could not install the new gear.

Thanks in advance. Bill
Click to expand...
Never heard that before, strange
 
My car has the same size tires.

originally it was a 2.93 rear and had a 31 tooth speedo gear

speedo was within a few mph ( maybe +/-2)

I swapped the rear to a 3.23 which called for a 33 tooth (based on above charts) but the speed was off more than I wanted. So I went to a 34 and it is in that +/- 2 again.

you will never get it 100% spot on, tires wear, the size is a nominal size.

if it's an automatic there is a percentage RPM increase due to converter slippage,

there are so many ways to calculate the correct gear and all are an approximation.

fyi

when I went from 2.93 to 3.23 that is a 9.3% difference

from 31 to 34 tooth is an 8.8% difference

that's as close as I'm going to get.



As others have said your speedo might be off internally.

a shop can spin it at a specific speed which will give you 1 mile per minute, then adjust the needle to be 60 mph.

the adjustment is deep in the head so nothing you can do on the fly.
 
My '69 Darts speedo is off too. Doesn't seem to care what speedo gear is in, pretty much reads the same. I will get it sent out one of these days.
 
Haven't read everyone's posts... So I might be repeating stuff.. Here goes..

First how is the odometer vs travelled distance... The odometer doesn't need the speedometer to be calibrated.... If the odometer is off your speedometer gear is wrong.... If the Odometer is accurate and the speedo is off the speedo needs to be calibrated...

The speedometer should read 60 MPH when the cable is spinning 1000 rpm
 
So if the tire is 781 revs per mile
And the rear is 3.91
The gear on the transmission is 13 teeth
And the speedo gear is 40

This should get a cable RPM of....

781×3.91×13÷40=992.5 RPM


So if the tire is 781 revs per mile
And if the rear is 3.73
The gear on the transmission is 13 teeth
And the speedo gear is 38

This should get a cable RPM of....

781×3.73×13÷38=996.6 RPM

You have tried both those gears and you are still off.

I think we need to nail down your rear gear ratio with some accuracy

But it's starting to look like your speedo is in need of calibration



(These numbers should get you to about 1000 RPM speedo shaft speed and as such 60 MPH (1 mile per minute))
 
1WildRT said:
Haven't read everyone's posts... So I might be repeating stuff.. Here goes..

First how is the odometer vs travelled distance... The odometer doesn't need the speedometer to be calibrated.... If the odometer is off your speedometer gear is wrong.... If the Odometer is accurate and the speedo is off the speedo needs to be calibrated...

The speedometer should read 60 MPH when the cable is spinning 1000 rpm
Click to expand...
Thanks I will check that out.
 
Dana67Dart said:
So if the tire is 781 revs per mile
And the rear is 3.91
The gear on the transmission is 13 teeth
And the speedo gear is 40

This should get a cable RPM of....

781×3.91×13÷40=992.5 RPM


So if the tire is 781 revs per mile
And if the rear is 3.73
The gear on the transmission is 13 teeth
And the speedo gear is 38

This should get a cable RPM of....

781×3.73×13÷38=996.6 RPM

You have tried both those gears and you are still off.

I think we need to nail down your rear gear ratio with some accuracy

But it's starting to look like your speedo is in need of calibration



(These numbers should get you to about 1000 RPM speedo shaft speed and as such 60 MPH (1 mile per minute))
Click to expand...
I hear you, I will know more this Thursday when I get a chance to take the car out again.
 
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