The speedometer on the 1973 Charger SE I just bought reads 20% slower than actual velocity at all speeds. The car originally came with a 318 cu. in. engine and 727 auto, which it still has. I don't know its rear end ratio or whether it has been changed.

Is there a way to figure out what what the correct number of pinion teeth should be to correct the speedometer's faulty reading without knowing the number of teeth on the existing pinion? (I assume I need a pinion with 20% fewer teeth than whatever is in the car currently, correct?)

I believe that there is a chart here on this site somewhere. You need to know your tire size and gear ratio though.

The speedo might be slow but how is the odometer? If the odometer is close to the actual distance traveled and the speedo is way slow the pinion is correct but the speedo needs repair.