72 Saginaw 3 speed Speedo Drive gear?

[72SB]

anyone know how many teeth the "speedo "drive" gear is?

6, 7 or 8?

is the drive gear size 1.84 or 1.76 OD?

its a 250 with 3:73 gears, all original drivetrain

[Accelo]

Do you have the original pencil gear (driven gear)?
All the gears, I have seen, on the output shaft have 8T but admittedly that doesn't mean much.

I guessed the tire size and used your given differential ratio (3:73).
This is the combination I came up with.

Differential ratio 3.73
trans shaft gear 8
Trans output gear 22
Assuming 205/75R15 tires
You would require the small gear.

The 1.84" OD gear uses 17 to 22 tooth pencil gears that are .810" OD
The1.76" OD gear uses 22 to 24 tooth pencil gears that are .870" OD.

Cheers

[72SB]

I have a 21 tooth now, driven gear. 20 (blue) would be better as speedo is off about 4mph at 55 mph as I now have 235/75 x 15 tires (28.9" tall)

It had a 22 tooth with the smaller tires I did have which I believe to be original size

I am doing a rear gear swap so will have to get another driven gear but was asking what the likely drive gear is in the Saginaw. I suspect 8 teeth but when I tried a 20 tooth it was too small or some reason it would not engage the 8-tooth drive gear so had to use the 21 which does work but is still off a few mph with the larger 75 series tires

My PG and a TH350 trans both have 8 tooth drive gears and I have several size driven gears for the various gear ratios I have had on those other 2 cars

[Accelo]

With your current size tires;
If you go to the 21T gear your spedomete3r will be off by 2.3% fast.
If you go to the 22T gear your spedomete3r will be off by 2.32% slow

You should have the smaller of the two 8T gears. (The 1.76" OD gear).

I get a 7% difference, in speed, using 55/59mph and your existing tire size and the 8/21T gear? Almost three times the what the calculated speed difference should be. Go figure.

[72SB]

Doing a rear gear swap and was asking what drive gear the Saginaw has which I assume is 8. With 3:08's, I will need a 17 tooth driven

with the 3.08's, my RPM will be 2500 at 70mph

[Accelo]

Your motor rpm will drop by 18% with the gear change.

For arguments sake lets say you installed a typical OD transmission.
You typically get a lower first gear that helps get you moving.
You get double the 18% percentage drop in overdrive. (35%)
If you add the cost of ring and pinion and possible a carrier change.
Then add the labor and compare it to the benefits of an OD.
Your budget and mechanical skills will give you an answer.
Cheers.