Anybody know which spedo gear is for the 3.42 rear?

And anyone know where I can get one? Do GM parts counters still have them?

 

Hopefully this will be a helpful site

HTH
Mickey

 

Same question for a 3.23 gear ratio?

Don't they go by color and teeth count? Available at a tranny shop if you know the right one?

 

Guys,

You didn't give year model(s) and my only parts book just covers back to '82, but I randomly picked a "drive gear"(in the tranny) #8634965 and a"driven gear" #25524925 (blue). Both these show to be available thru gmpartsdirect.com
In the end (for best accuracy) you'll also need to know tire diameter of course.

HTH

Mickey

 

One more thing and I'll leave you alone :grin:

Speedometer Gear Formula
Number of Driven Teeth = (Number of Drive Teeth x Axle Ratio x Tire Revolutions per Mile*) / 1001

I know...you want this too.

*Tire Revolutions per Mile Formula (TRPM)
TRPM = 20168 / Tire Diameter (in inches)

Have some fun

Mickey

 

jeff do you need the plastic pinion gear for the speedometer for a turbo 400 ? if that what you need, let me know, i have a few nos one and may be able to help.

henry white

 

Here is some useful info I found on the gnttype.org web site, hopefully the formatting will hold. If not, you can go

NOTE: Each tire's section width is based on a particular rim size and construction. Compensate for differences. Increase section width .2 inches for every .5 inch increase in rim width. For example, if the tire's section width is 9.14" on a 6.5" rim and is to be mounted on a 7" rim, the section width would be 9.34". Section width (sidewall flex) will vary by construction. ANY variance will affect calculations!

Radius - Diameter - Circumference

To compute the diameter of a P-Metric tire, multiply the section width by the aspect ratio (as a percentage). If necessary, divide this number by 25.4 to convert millimeters to inches. For example, a 215/65 equates to a tire height of 5.5019685 inches (215 x .65) divided by 25.4. Multiplying this by 2 for the top and bottom sidewall height and adding the rim size will get the overall tire diameter. For example, if the tire is on a 15 inch rim, it is approximately 26 inches ((5.5 x 2) + 15). The radius is half of the diameter. Multiplying the diameter by pi (3.1415927...) will get the circumference.

Tire Revolutions Per Mile



To compute the number of tire revolutions per mile, divide 20,168 by the diameter.

20168
-------------------------------------- = Tire Revs/Mile
(Tire Heigth X 2) + Rim Size


For a 215/65R15:

20168
---------------------- = 775
(5.5" X 2) + 15


For a 255/60R15:

20168
------------------- = 747
(6" X 2) + 15

Speed

By dividing the original tire revolutions by the new tire revolutions and multiplying by the indicated speed on the speedometer, we arrive at the new actual speed.


(Orig. Tire Revs/Mile)
------------------------------- X Indicated Speed = Actual Speed
(New Tire Revs/Mile)

Using the above tire sizes:

(775)
-------- X 60 mph = 62 mph
(747)

Rear Axle Ratio

Changing tire diameters will also change the effective rear axle gear ratio. Divide the new tire revolutions by the original tire revolutions and multiply by the original axle ratio. For example, a turbo Buick that had 215/65s and a rear axle ratio of 3.42 (what else?) and moved to a 255/60 will net an effective ratio of 3.30.


(New Tire Revs/Mile)
---------------------------- X Orig. Axle Ratio = Effect. Axle Ratio
(Orig.Tire Revs/Mile)

(747)
-------- X 3.42 ratio = 3.30 ratio
(775)

As you can see, this will theoretically raise the top speed of the vehicle but
hurt its acceleration.

To regain the original effective ratio with the new tires, a 3.55 ratio is
required.

(Orig.Tire Revs/Mile)
------------------------------ X Orig. Axle Ratio = Equiv. Axle Ratio
(New Tire Revs/Mile)



(775)
--------- X 3.42 ratio = 3.55 ratio
(747)

Maximum Speed

To calculate the maximum speed of a vehicle, take:

Overall Tire Diameter X Achieved Engine RPM (redline)
----------------------------------------------------------------------------- = Max Speed
Differential Ratio X Top Gear Ratio X 336

26" X 5250
--------------------------- = 177 mph
3.42 X 0.67 X 336

This differs from actual speed. A stock turbo-Buick's actual top speed is in the
152 to 162 mph range. The aerodynamics on these cars are nothing to write home
about.
here

 

More good info in this Previous Thread on the subject of speedo gears:TU:

 

3:42 speedometer gear

Has anyone figured out the correct gear for a 3:42 ratio? I have 255-60-R-15s on my GS455 and a 3:42 posi and like to know what speedometer gear to use in my Muncie, Thanks AL

 

Keep in mind -

A radical change in driven gear (the one the cable slips into) will require a different carrier hub (piece you take out to change the gear) in the transmission. The gear sizes are cast into the hub. Some carriers are hard to find.

The drive gear on the transmission tailshaft came in (2) styles - a pressed on steel gear and a snap in place nylon. Removing the steel gears is REALLY tough sometimes. Just finding a shop with the right tool is hard. Change to the nylon gear if you have to take the steel gear off.

 

GEEZ!!!ou: I didn't know it would be so complicated.

I never look at the speedo anyway (because of the gauges on the column I can only see between 30 and 50 MPH anyway).

If I ever get the hankerin' to stuff myself up uder my Z-frame and count my teeth, I'll revert back to this thread.

Thanks for the input everyone.

 

I always just went to the GM parts counter

Told them the model yr, trans, rear gear, and tire size. I don't ever recall having any problems....But it was 20 yrs ago since I needed one.

 

Too much math for me. I think I will just continue to believe that I can turn 140 trap speeds!

 

ROFLMAO!!!!!!!!!!!!

:laugh: :grin: :laugh: :grin: