Air idle adjustment. Mechanics when in doubt will always tell you your intake manifold somewhere around the carburetor / or fuel injection system on top of the intake manifold has a gasket leak but I've found there are other reasons and found my high idle fix at the air idle adjustment on the side of a throttle body injection system on my C1500 pickup big block chevy. To my disappointment the mechanic that looked the most educated in a huge dealership service facility with his head full of white hair told me I had a friggon leak when I instantly fixed the problem after the ten minutes of work at the air idle adjustment on the side of the injection that I was only able to find because I bought my trucks service manual specific to my truck.
Water levels need to be high
Rpm`s are how many rotations are in a minute so are you reving your engine?
Your gear ratio is to low for your driving speed