How many teeth?

[younghoss]

Just bought '73 Duster, 340. Speedo says 80 when I'm really going about 40. Mechanic friend says likely due to wrong speedo gear. How do I determine the right one?
Looks to me like there's a formula to compute using tire diameter and axle ratio. I want to restore just as it came out of the factory. I know tire diam is 25.3 but where can I find what axle ratio was from factory?

 

[69FBCuda]

By turning the tire 1 rotation and counting the driveshaft rotation or see if there is a tag bolted to the center section.

 

[younghoss]

Thanks!

 

[Dana67Dart]

By turning the tire 1 rotation and counting the driveshaft rotation

If its an Open diff keep one tire on the ground. If Sure Grip both wheels off the ground.

Use a GPS, Garman, Phone to drive at 60 mph then record what the speedo says.

My bet is that there is something else going on here and your 40 = 80 is not accurate enough because that says your speedo gear is about 2x as small as it should be, hard to believe.

Something else that can effect this is the speedo internals, but do the easy stuff first.

 

[AJ/FormS]

The easiest way to get 100% accuracy is to figure the exact error in percentage, then transfer that error to whatever tooth count is in your trans.
In your case, a reading of double is hardly logical. If you look at the chart, the minimum tooth count is 21, and to slow your speed-o down to half would require a tooth count of 42 . While this scenario is possible, it is hardly likely.
So the most logical answer is that under your car, someone has installed a speed-O doubler. so go look. There should not be any adapter in the speed-o cable, anywhere in it's length. If there is. just take it out, and roadtest again. Those adapters come in at least two ratios that I have seen.

 

[junkyardhero]

if its not the speedo gear, or a ratio adapter scabbed on there, it's possible the clock spring in the speedo ***'y is broken or some other type of internal failure of the gauge itself.

 

[TrailBeast]

if its not the speedo gear, or a ratio adapter scabbed on there, it's possible the clock spring in the speedo ***'y is broken or some other type of internal failure of the gauge itself.

I have to agree.
40 mph off isn’t something a gear change is going to fix.
Something is wrong with the speedo head.

 

[younghoss]

thanks guys