727 speedometer cable leaking

As Snake said there are 2 seals and it can leak from either but the most prominent cause is the internal seal. When you have to take the speedometer pinion adapter out to replace a seal your better off replacing them both because even if it's just one leaking R&R'ing may cause the other to leak. Go to your favorite parts supply and buy a factory replacement outer seal, make sure it's orange as it's a special seal that ATF won't attack and buy an inner seal for a early to mid 70's GM turbo 400 (if they need an application tell them it's a 73 Chevy truck with a 454 engine and turbo 400 trans). It's the same dimension but has an inner spring to hold it tight against the pinion shaft. Mopar seals don't have the inner spring and are more prone to leaking. When you install them lube them up good with trans. assy. lube (Vaseline works too if you have no assy. lube) and install it in reverse order of removal. Here's a pic I modified showing the seals and the inner one that is problematic. Also make note of the clocking of the adapter housing before removal so you get it back in correctly. What I've found that works the best is to install it with the largest #'s located at the 6 O'clock position then rotate it until it lines up where it's supposed to be, assuming of course it needs to be on one of the positions with a lower set of numbers. Those #'s correlate with the tooth count of the speedo pinion gear.