Stupid rear end questions...

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808-dartGT

808-dartGT

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Guys, I wanted to check what was going on with oil and wear my 7.25 rear end. It has some kinda massive torx plug. I have a manual, but it doesn’t say anything about the size or type of tool I need...
another question is I noticed that my speed is higher than what is on speedometer (those digital speeding displays on the side of the roads) if I go around 30, actual speed is around 35mph or so. So what I did I counted tire vs driveshaft revolutions and I believe I have 2.76 gears. I have ‘64 dart GT 225 4speed , in my manual says gears were 3.55 standard and 3.23 optional. So does that mean gears were swapped at some point? Where I could get plastic gears for transmission to fix speedo error?

thank you very much!
 
808-dartGT said:
So I measure actual tire height and numbers in the graph is how many teeth on a Speedo gear?
Click to expand...

you need to know exactly what gear in in the car and the height of the tire...


here is a for instance. i just went from a 2.94 gear to a 3.23 gear...

26" tire.

2.94 uesed a 30 tooth (small one in the picture) speedo gear..

3.23 uses a 33 tooth (larger one in the picture) gear.

speedo is dead on with my GPS


if you do change the speedo gear make sure you clock the speedo gear housing properly on install..

IMG_20200329_130222.jpg


IMG_20200329_130334.jpg
 
808-dartGT said:
I noticed that my speed is higher than what is on speedometer (those digital speeding displays on the side of the roads) if I go around 30, actual speed is around 35mph or so.
Click to expand...

Could be as simple as different tire height than what came on it from the factory.
 
Remove plug, stick your finger in the hole. If you feel gear oil, you're good.
If not, fill it with gear oil until it runs out the hole. Then reinstall plug.
 
If your actual axle ratio doesn't match what the car originally came with I can almost GUARANTEE the original failed and was replaced with a used 3rd member at some point.
Those 7 1/4's are weak at best. Our '66 Cuda had 4 between 1966 and 1982 and it is an auto.
Yours is a 4 speed I see?
Guarantee it broke a few too over the years.
Not saying I would replace it immediately or anything, but the first time it needed anything besides 90W I would plan on an upgrade to something else.
A lot of that depends on YOUR driving habits of course!!
 
808-dartGT said:
I have ‘64 dart GT 225 4speed , in my manual says gears were 3.55 standard and 3.23 optional. So does that mean gears were swapped at some point?
Click to expand...

It’s a possibility that the whole original rear end was swapped by a PO rather than just gears replaced. I’m willing to bet that your 4 speed is original to your 64, which means 66 and later Speedo pinion gears will not fit as suggested by several members. Here is what the business end with the 65-earlier pinion gear looks like:

0A8D01DE-3661-42C9-9D56-F6BED5FB3E72.jpeg


Speedometer pinion gear sizes were based on tire size and rear end gears. Here is the tire size/pinion gear chart for 65; for the A Body the only tire sizes available were either 6.50x13 or 7.00x13. The 14 and 15inch tires were only for the B Bodies and full size boats. You can probably use the smaller 14 inch tire size to figure the speedo pinion size you need if you’re running 14 in tires. Notice that with the 833, the 2.76 geared rear end was not available!

A492FC6D-F405-4710-980A-B5077541C670.jpeg


“808-dartGT said:
Where I could get plastic gears for transmission to fix speedo error?

thank you very much!
Click to expand...

They are not available new so used will be it. After you figure out which one you need, you might try Brewers. Otherwise post a Want Ad for the specific size you need. Hope this helps
 
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65 Dartman said:
It’s a possibility that the whole original rear end was swapped by a PO rather than just gears replaced. I’m willing to bet that your 4 speed is original to your 64, which means 66 and later Speedo pinion gears will not fit as suggested by several members. Here is what the business end with the 65-earlier pinion gear looks like:

View attachment 1715497577

Speedometer pinion gear sizes were based on tire size and rear end gears. Here is the tire size/pinion gear chart for 65; for the A Body the only tire sizes available were either 6.50x13 or 7.00x13. The 14 and 15inch tires were only for the B Bodies and full size boats. You can probably use the smaller 14 inch tire size to figure the speedo pinion size you need if you’re running 14 in tires. Notice that with the 833, the 2.76 geared rear end was not available!

View attachment 1715497578



They are not available new so used will be it. After you figure out which one you need, you might try Brewers. Otherwise post a Want Ad for the specific size you need. Hope this helps
Click to expand...
So being that I have 64, I feel special again...
 
motorpirate said:
If your actual axle ratio doesn't match what the car originally came with I can almost GUARANTEE the original failed and was replaced with a used 3rd member at some point.
Those 7 1/4's are weak at best. Our '66 Cuda had 4 between 1966 and 1982 and it is an auto.
Yours is a 4 speed I see?
Guarantee it broke a few too over the years.
Not saying I would replace it immediately or anything, but the first time it needed anything besides 90W I would plan on an upgrade to something else.
A lot of that depends on YOUR driving habits of course!!
Click to expand...
I take it easy on this one, at least while I’m working on it. Yesterday I did rear wheel bearings and since I had him on a stands I did the count for ratio. I like tall gears for economy and cruising speed, but wish speed was correct.. I really dot want to swap rear end...
 
I just looked at my tire size , I have 14” rims and tire height measures 23” brewers graph table starts with 24” lol
 
That plug on mine was stripped and a bugger to get off. Once I finally got it off I replaced it with a bolt end type.
Good luck
 
So I measure actual tire height and numbers in the graph is how many teeth on a Speedo gear?
Click to expand...
last 2 digits in the pinion # is the # of teeth. Jack 1 wheel off the ground/trans in neutral/stem at 12 O'clock/rotate tire 1 revolution/count # of shaft turns/divide by 2. 2.76 is 2&3/4 turns/3.23 is 3&1/4 turns/3.55 is 3&1/2 turns. measure height with tire at correct psi. jack it up high on the drivers side to minimize fluid loss. A GPS would be nice as you could get the exact % of error & that is the % of teeth change you need (less teeth in your case to speed it up) tho that would mean pulling it to check the teeth # then sourcing the correct tooth pinion then jacking it up again & pulling it again (a mess) where as going by the chart would entail doing it only once tho the chart might be slightly off but I used it & it came out dead on. If I had to choose between 2 close teeth selections I would the speedo to read a hair high (could prevent a ticket)
 
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Rapid Robert said:
last 2 digits in the pinion # is the # of teeth. Jack 1 wheel off the ground/trans in neutral/stem at 12 O'clock/rotate tire 1 revolution/count # of shaft turns/divide by 2. 2.76 is 2&3/4 turns/3.23 is 3&1/4 turns/3.55 is 3&1/2 turns. measure height with tire at correct psi. jack it up high on the drivers side to minimize fluid loss. A GPS would be nice as you could get the exact % of error & that is the % of teeth change you need (less teeth in your case to speed it up) tho that would mean pulling it to check the teeth # then sourcing the correct tooth pinion then jacking it up again & pulling it again (a mess) where as going by the chart would entail doing it only once tho the chart might be slightly off but I used it & it came out dead on. If I had to choose between 2 close teeth selections I would the speedo to read a hair high (could prevent a ticket)
Click to expand...
I did turns , I get somewhere around 2.8, so it can be 2.76 or 2.94? I have 21 tooth speedo gear, I talked to Wayne from Brewers, I’ll try to get speed with GPS and see what is happening. I think getting speedo reading high is not a bad idea
 
Make it simple; calculate the speed error in percent. That is the same percent you need to change on your trans gear.
If your speed-O is reading hi, then you need to slow the cable down.
If reading low, the cable has to spin faster.
Do this by adding or subtracting teeth to the Speed-O gear; whatever percent you calculated.
In your case;
35/30 =plus 16.7%, and you need to speed it up, so you need 16.7% fewer teeth. and from 21 that would be;
(1 less 16.7%) times 21=17.5, which you can round up or down.

BTW
As you can tell by the varied answers; this is NOT a stupid question.
Doing it by percent, gets you to within a half a tooth every time. By the chart, yur kindof guessing, cuz modern tires are metric and there is no accurate way of translating between the two systems.
Whereas math is just math, just get your percent error as close as you can, and the faster you are moving the closer will be the result.
For instance, you say at 30 actual mph, your speed-o is reading 35, a 17.6666% error. If you increased your speed to 60, you would expect the same 17.66% error. but what if your speed) now reads 69, then the new error is 69/60= 15% and your new gear is (1 less 15%)x 21=17.85 teeth. So now you round to 18 teeth. Whereas earlier, you had 17.5 which could round either way.
 
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AJ/FormS said:
Make it simple; calculate the speed error in percent. That is the same percent you need to change on your trans gear.
If your speed-O is reading hi, then you need to slow the cable down.
If reading low, the cable has to spin faster.
Do this by adding or subtracting teeth to the Speed-O gear; whatever percent you calculated.
In your case;
35/30 =plus 16.7%, and you need to speed it up, so you need 16.7% fewer teeth. and from 21 that would be;
(1 less 16.7%) times 21=17.5, which you can round up or down.

BTW
As you can tell by the varied answers; this is NOT a stupid question.
Doing it by percent, gets you to within a half a tooth every time. By the chart, yur kindof guessing, cuz modern tires are metric and there is no accurate way of translating between the two systems.
Whereas math is just math, just get your percent error as close as you can, and the faster you are moving the closer will be the result.
For instance, you say at 30 actual mph, your speed-o is reading 35, a 17.6666% error. If you increased your speed to 60, you would expect the same 17.66% error. but what if your speed) now reads 69, then the new error is 69/60= 15% and your new gear is (1 less 15%)x 21=17.85 teeth. So now you round to 18 teeth. Whereas earlier, you had 17.5 which could round either way.
Click to expand...
That’s actually very good explanation with math! I have to put my front wheel together after wheel bearings come and go on the freeway
 
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