What would original speedo gear have been for '70 Charger?

[Coelacanth]

Just wondering what the number of teeth original speedo gear would've been for a 1970 Charger, 383, 727 Auto and whatever the original tire size was? My car had P195-75-14's when I bought it in 1986 but those wouldn't have been the original tires, obviously. I want to compare number of teeth recommended when changing to 28" tires and 3.55 or 3.73 gears. I would guess all the 727's with the original tires and 3.23 rear ends would've had the same number of teeth speedo gear from the factory?

It looks like a 33-tooth yellow speedo gear would be the correct gear for a 3.55 gear swap and 275-60-15's, just wondering how that compares to whatever was stock.

 

[Darter6]

Year One has a chart and it looks like you are correct with yellow 33 tooth.
Green is a 34 and red is a 35. looks like the 33 is as close as you can get. Most of the time you will be within 4 mph what ever you choose.

 

[Car #4]

If you’re doing a gear and/or tire swap don’t buy a new speedometer gear right away. Do your swap and then download a free speedometer app and go for a drive. Figure out the percent difference between the app and your speedometer as a second opinion to what the chart tells you. It’s just a way to double check before you go spending money on a new speedometer gear only to find out you still aren’t accurate.

 

[toolmanmike]

Brewer also has a chart.

 

[Coelacanth]

Thanks, I've seen that chart, but does anyone know what the OEM speedo gear tooth count would've been? Without disassembly of the original, that is?

 

[toolmanmike]

Thanks, I've seen that chart, but does anyone know what the OEM speedo gear tooth count would've been? Without disassembly of the original, that is?

I don't think I have ever seen it mentioned or in print. Possibly buried in a service manual with a chart for different sized tires that were available and rear gear ratios.

 

[Coelacanth]

If you’re doing a gear and/or tire swap don’t buy a new speedometer gear right away. Do your swap and then download a free speedometer app and go for a drive. Figure out the percent difference between the app and your speedometer as a second opinion to what the chart tells you. It’s just a way to double check before you go spending money on a new speedometer gear only to find out you still aren’t accurate.

That's something along the lines of what I was thinking. Maybe the OEM gear and smaller diameter 26" tires with a 3.23 rear end might be offset similarly with 28" tire size increase and 3.55 gear ratio decrease. If I knew the original gear, the math would be easy.

 

[Jerry Hall]

Original tire diameter was al 25.3" (775-14) so a 33 or 34 tooth speedometer gear are as close as you can get with a 3.23 gear.

 

[Coelacanth]

So the original speedo gear might be 33 or 34 teeth, with original tires and a 3.23 rear end?

 

[R413]

It is on the broadcast sheet under pin or pinion.

 

[Coelacanth]

Would that it could be so simple...my broadcast sheet is firmly stuck to the springs and only a complete and exceptionally careful disassembly will let me remove it without major damage...

 

[Jerry Hall]

Would that it could be so simple...my broadcast sheet is firmly stuck to the springs and only a complete and exceptionally careful disassembly will let me remove it without major damage...

View attachment 1715658

My 67 GTX 3.23 sure grip 440 automatic. Build sheet says pinion 32

 

[69Coronetrt]

I don't think I have ever seen it mentioned or in print. Possibly buried in a service manual with a chart for different sized tires that were available and rear gear ratios.

The charts are posted in the parts books. Here are the '70 and '71 charts. Gear ratio and tire size = pinion number.

 

[Coelacanth]

Thanks for the chart. I'll have to take a closer look at my broadcast sheet and see if I can read it if it's not obstructed by the seat springs...

 

[Car #4]

It not hard to pull the gear and see what you have now.