77 Auto Trans with RED Speedo gear??

[XQIZT]

Pulled the speedo gear and it is in fact red (21 teeth). According to the Eckler's catalog, the red 21 tooth gear is only for 4 speed trans. Auto choices are a blue 20 tooth gear or a natural 19 tooth gear.

SOOooo, either 77 hs some kinda special transmission OR someone just jammed in any kind of gear they could find.

Any way to determine the correct gear to use?

[XQIZT]

ORRRRRRrrrrrrrrrrrrr it's also possible that there IS a red 21 tooth gear for the auto trans, but Eckler's doesn't list it. Corvette Central offers one.