71monacomoparmechanic
Member
Changed 2:76 to 3:55. What is the method of making the spedometer correspond? Rear is 489 8.75 and 727 torqueflite. 383 engine.
Changed diff gear. How do i adjust speedometer?
- Thread starter 71monacomoparmechanic
- Start date
If you look in the parts book, there are different drive gears that are different colors for the various gear ratios. Not sure how many are still available though.. You just pull the speedo drive from the transmission and insert the correct drive gear to correct for a different rear end ratio.
Dave
Find the FSM for your car. I believe in it, it'll list the speedometer gears you need for the transmission, but if you look in a Chrysler manual, it might not go to 3.55, but just to 3.23. Possibly Plymouth or Dodge C-body manuals get to the 3.55?
"Drive" gear slides onto the output shaft of the transmission. Slides on and clips. "Driven" gear is in the housing where the cable attaches. Remove the housing and the gear slides out, but to get to the drive gear, the rear tailshaft housing of the trans must be removed and reinstalled.
OTHERWISE, you can usually find a "adapter gearbox" to make up the difference without having to put new speedo gears in the trans. GM used a bunch of those! 'Ratio adapter", most are "offset", but others are "right angle" in configuration, depending upon floor pan clearance issues.
With the deeper gear in the rear end, you'll need to slow down the speedo cable. 2.76 divided by 3.55 would probably be the ratio you need. Aim for distance accuracy rather than speed accuracy. The distance accuracy means the total speedo gear is where it needs to be, or very close. Getting that relationship back to formerly-normal will mean the factory calibration of the speedometer speed magnet is still good.
The KEY is going to be to find a speedo shop that can still custom-build the ratio adapters. Might end up in a hd truck-related place than a car-related place, unless you can get to an old-line speedometer shop.
Enjoy!
CBODY67
"Drive" gear slides onto the output shaft of the transmission. Slides on and clips. "Driven" gear is in the housing where the cable attaches. Remove the housing and the gear slides out, but to get to the drive gear, the rear tailshaft housing of the trans must be removed and reinstalled.
OTHERWISE, you can usually find a "adapter gearbox" to make up the difference without having to put new speedo gears in the trans. GM used a bunch of those! 'Ratio adapter", most are "offset", but others are "right angle" in configuration, depending upon floor pan clearance issues.
With the deeper gear in the rear end, you'll need to slow down the speedo cable. 2.76 divided by 3.55 would probably be the ratio you need. Aim for distance accuracy rather than speed accuracy. The distance accuracy means the total speedo gear is where it needs to be, or very close. Getting that relationship back to formerly-normal will mean the factory calibration of the speedometer speed magnet is still good.
The KEY is going to be to find a speedo shop that can still custom-build the ratio adapters. Might end up in a hd truck-related place than a car-related place, unless you can get to an old-line speedometer shop.
Enjoy!
CBODY67
Change the speedo gear to what you need .every gear has a different tooth count. Its on the side of the transmission where the cable is attached . The gears are for sale on EBay. No adapter needed .
The FSM will take tire size into the mix, as that's an integral part of the whole equation. Some might have difficulty crossing the old sizes into the newer sizes, though. Just a normal part of things as tire sizes have changed over the years.
The housing which holds the driven gear has some numbers on the side of it. The hole is not exactly in the middle, but has a slight offset to it. To progressively accommodate driven gears with more teeth and/or a larger gear diameter, as it rotates in the tailshaft housing. So, something else to look for and pay attention to, by observation. ALSO, the tube the driven gear slides into CAN be a wear point in the system. So look for any change in that hole from completely round. Plus be sure to put some lube on the new gears' teeth and also on the driven gear shaft prior to putting it into the housing it goes into. A bit of lube on the seal of the driven gear housing is good to do, too. Just a dab so it slides in smoothly. Note the index mark vs the number of teeth (range) cast into the housing itself.
Just some observations,
CBODY67
The housing which holds the driven gear has some numbers on the side of it. The hole is not exactly in the middle, but has a slight offset to it. To progressively accommodate driven gears with more teeth and/or a larger gear diameter, as it rotates in the tailshaft housing. So, something else to look for and pay attention to, by observation. ALSO, the tube the driven gear slides into CAN be a wear point in the system. So look for any change in that hole from completely round. Plus be sure to put some lube on the new gears' teeth and also on the driven gear shaft prior to putting it into the housing it goes into. A bit of lube on the seal of the driven gear housing is good to do, too. Just a dab so it slides in smoothly. Note the index mark vs the number of teeth (range) cast into the housing itself.
Just some observations,
CBODY67
71monacomoparmechanic
Member
Thanks. Informative
After I say a censer welcome to the FCBO Party, I'll add ah couple thoughtz 71Monaco. The pic that welder guy showed you iz of ALL the worm gearz for the late '60s and early '70s 727s so you can match your worm gear to one or the other for size of those sets of gear sizes. Thought #2, I'd suggest that you hang on to the 276 worm gear and Punkin' 'cuz after you get tired of smokin' tires and pullin' stumps and averaging 7-8 MPG you might want to re-think the 355 gear. OH? And Happy Trailz, Jer
71monacomoparmechanic
Member
Yea looks like i can just put a 32 tooth gear in and will be ok. I dont have 2 differential housings i just changed the ring and pinion. It doesn't seem to be running too high of rpm. That 276 gear was just too high making that car accelerate on the slow side. I don't drive the daily. I'm saving the old ones anyway
On my '66 Newport, I had better "stop light grand prix" results from adding two more turns of preload into the kickdown linkage to raise the automatic shift points a few mph, rather than manually shifting from 1-2 and 2-3. That was in '74. Now, almost any little 4cyl car with a "zillion-speed" automatic can leave you in the dust.
With H78x14 tires, the 2.76 should be about 28.66/1000rpms in high gear. With H78x15 tires on my '70 Monaco and 3.23, it was about 25.4mph/1000 rpm. With a 3.55, that might drop to about 24.5mph/1000rpm. So, 2000rpm goes from about 57 mph down to about 49mph with the 3.55s? Not a lot of perceived rpm differences for the same cruise speed, at least up to about 70mph. Throttle response might be a bit better, though. Key is to use "just enough" rather than otherwise.
Just some thoughts,
CBODY67
With H78x14 tires, the 2.76 should be about 28.66/1000rpms in high gear. With H78x15 tires on my '70 Monaco and 3.23, it was about 25.4mph/1000 rpm. With a 3.55, that might drop to about 24.5mph/1000rpm. So, 2000rpm goes from about 57 mph down to about 49mph with the 3.55s? Not a lot of perceived rpm differences for the same cruise speed, at least up to about 70mph. Throttle response might be a bit better, though. Key is to use "just enough" rather than otherwise.
Just some thoughts,
CBODY67
71monacomoparmechanic
Member
I think the 3:55 is good. I originally bought 3:73 ring and pinoin but the spline on the pinion didnt match up to the u joint yoke.
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