RPM drops around 50 mph

Hi - I am Dave, contributing to RepairPal and providing expert advice since 2010. If turning off the overdrive gear seems to help, then it would sound to me like you have a transmission issue. Diagnostic testing must be performed with the engine performance and transmission operation areas. With testing comes a diagnosis and direction for repair. Start there. https://repairpal.com/auto-repair-near-me Click the link to find some shops near you who can help out!