pcard

I am thinking of changing the ratio on my 1970 Cutlass from 3.08 to 2.78. I know this will require a change in the speedometer "driven gear" mechanism on the transmission. As one would think, in reviewing the Assembly Manual section 7, I see that the specific part is dependent on the tire code as well.
Can any advise on what tire diameters correspond to the codes: 30, 33, 35,10, 15, 20, 23,25, 21, 26.

Also, does anyone know where I might find a new speedo "driven gear"?

thanks very much.

Run to Rund

For the same tire size, the number of driven gear teeth will change as follows:
(2.78 / 3.08) x (original number of teeth).

joe_padavano

Depends on the trans, but here's one source:

http://www.transmissioncenter.net/sp...n_______va.htm

pcard

Hey Joe - thanks alot!
Run to Rund - this would work if I knew what the tire diameter was. Once I know the tire diameters of the different codes above, then I can look up what driven gear I need for my tires.

svnt442

This might help. you can do some conversions I believe. Generally it's assumed the car had the equivalent of a 225/70/14 which is 26.4" in diameter. Another thing is if the speedo is accurate now you can always reference the chart to see what you have for drive and driven gears to give yourself a starting point to go off of.

misc.jpg

pcard

Hey Randy - thanks. My build sheet indicates that I had the 42 tooth gear, and the G78X14 tires. In doing some research I came accross this web site ( http://carnut.com/specs/tires.html ) which gave me the nominal width of the G78x14 as 27.1 inches, and F78x14 with a width of 26.5. It is interesating that your chart above gives these two tires the same size of driven gear. So obvioulsy they are at an upper and lower range of tire size for a specific speedo driven gear. So using your chart if I go to the 2.78 rear end ratio and keep a 26.5 -27.1 inch tire then I will need to get the 38 tooth blue driven gear, which in 1970 is part number 1359272.
Then I looked up the details on current radial tire specs, and in wikipedia (http://en.wikipedia.org/wiki/Tire_code ) I find the formula: D = (X * Y)/1270 + Z, where D is the diameter in inches, X is the tire width in mm, Y is the aspec ratio nd Z is the wheel size in inches. So, for example the P225/70/14 has a diameter of 26.40 inches, the P235/70/14 has a diameter of 26.95 inches (two decimal places). (Or I could have looked it up in the chart on the website - duh.)
So I imagine that I can go with either of these tires, with the 235/70/14 being a closer match to the original G78x14 diameter.
All this could have been avoided by just considering that droping from 3.08 to 2.78 is a 9.74% reduction, and so I would go from 42 teeth to about 38 teeth. But it has been a fun exercise anyway, and I learned some stuff.

pcard

I guess they do not make the 235 anymore. I will have to go retro with the G78 ww from BFG.

defiant1

Need some help.

Based on the calculation below I think I need a 42 tooth driven gear. http://www.bgsoflex.com/speedo1.html
The only thing I don't know is the Speedometer revolutions/mile for a 1970 Cutlass. So my equation may be off.

I have 275 60R15 tires (28" diameter), TH350, and 3.23 differential. I used to have a 2.56 (R2 code).


Input Parameters Are the Following:

• Speedometer Revolutions/mile = 1000
• Rear Tire Diameter (Inches) = 28.00
• Differential Gear Ratio = 3.23
• Drive Gear Teeth Number = 18

Computation Results:

Computed Driven Gear is 42
Ratio of Driven Over Drive Gear is 2.3

pcard

I concur, you are looking at a 42 driven gear. The 275/60/15 has a diameter of 27.99 inches. You are close to the 855/14 tire which is 28.1 inches and from the chart above requires the 42 gear.
The next gear, 43, matches up with a H78 with a diameter of 27.8 inches. This would give you a slight underreading of your speed - not a good thing.

defiant1

Awesome, right now my speedometer reads 20 mph higher than actual speed.

The 42 tooth will mate correctly with the TH350 drive gear??

d1

pcard

You will probably need to change the driven gear housing. I would give these guys a call. They will have your gear as well.

See: http://www.transmissioncenter.net/sp...n_______va.htm

svnt442

Yes it will. And that 20 mph is not constant, it changes the faster you go.

defiant1

Good to know, I will get one on order. The MPH not constant...hmmm I guess that's why the speedo needle was buried before I even gave 'er hell.

jpc647

How did you determine 1000 speedometer revolutions per mile? And did it come out accurate when you replaced the gear.

defiant1

I used 1000 revolutions per mile as the standard calibration found in most speedos.
After the install, the speedometer reads approximately 3 mph faster than actual speed.

d1

jpc647

Thanks. Did you have to buy a new housing as well, for the 42 tooth gear? Did you simply replace the o-ring with another one, or did you use one of the "speedometer bullets". Sorry for all the questions, just want to do it right.


Also svty442, Where did you get the chart in your post?

defiant1

I got a new housing. Frustrating that you have to do that, but it did the trick.