I am new to this site so please bear with me. There is a leak around the speedo gear and my mechanic says I need a new speedo gear, but he doesn't know which type. The car is a 1970 442 (not a w30)with a big block 455. He says there are two sizes, one with a long shaft and one with a shorter shaft. How do I determine which I need? Thanks for any help you can give.

 

Automatic trans I'm guessing...

The rear end will determine what the gear will be. It's really easy to figure out what is in there now. All you need to do is unscrew the speedometer cable from the trans and the remove the bolt that holds the gear housing in the trans. Then the housing should come out with little difficulty. It MIGHT need some minor persuasion if it has never been out of the trans, but it shouldn't be bad.
All you need to know is the tooth count and that you have a TH400 trans and getting the correct gear is a breeze.

 

 

The color of the gear indicates the tooth count.

 

Welcome. It's in your best interest to start a new thread on your problem rather than tack your question onto a completely unrelated thread about a completely different car and transmission. I've moved your question to it's own thread.

As noted, we need to know which trans you have. Note that not only can the plastic gear be worn, but the seals in the housing that holds the gear can also be bad. There are TWO such seals, on in the inside diameter that seals against the gear shaft and one on the outside diameter that seals the housing to the trans case.

 

Thanks to svnt442 and Joe Padavano. You have both helped more than you can know. I am not capable of doing this myself but I have printed off both posts and photos and will take them to the mechanic. I do appreciate it.