Start with some vehicle info and background.
With those big tires on a 2.5 without knowing your axle type or the type of road its only a guess here. If you're topping out at 50 then it us safe to assume you're still stock and that's all she has to offer. I bet she has no acceleration? So thus leads to either changing gears/axles or smaller tires.
The idle issue could be alor if things. We'll need to know make model year engine transmission.